Upgrade applets, desklets, themes, etc
Update Update Manager, restart update manager
apply all updates
select mint upgrade from edit menu

apt install p7zip-full
apt-get remove xul-ext-lightning

xscreensaver hack no longer supported on cinnamon:

apt remove --purge xscreensaver-data xscreensaver-data-extra xscreensaver-gl xscreensaver-gl-extra cinnamon-screensaver-webkit-plugin cinnamon-screensaver-x-plugin

reboot

sudo apt-get purge libflatpak0 gir1.2-flatpak-1.0 

apt install mintupgrade
apt autoremove
mintupgrade check
- disable third party repos/ppas (software sources)
- downgrade foreign packages (software sources - maintenance)

sudo apt-get install kodi=2:17.6+dfsg1-1ubuntu1 
sudo apt-get install init=1.51 iproute2=4.15.0-2ubuntu1.1 libopenmpt0=0.3.6-1 libopenmpt-modplug1=0.3.6-1 gir1.2-networkmanager-1.0=1.10.6-2ubuntu1.4 gir1.2-nm-1.0=1.10.6-2ubuntu1.4 libnm0=1.10.6-2ubuntu1.4 libnm-glib4=1.10.6-2ubuntu1.4 libnm-util2=1.10.6-2ubuntu1.4 network-manager=1.10.6-2ubuntu1.4 network-manager-config-connectivity-ubuntu=1.10.6-2ubuntu1.4 skypeforlinux=8.59.0.77

mintupgrade download
mintupgrade upgrade

cat /etc/apt/sources.list.d/official-package-repositories.list
# Do not edit this file manually, use Software Sources instead.

deb http://packages.linuxmint.com ulyana main upstream import backport #id:linuxmint_main

deb http://archive.ubuntu.com/ubuntu focal main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu focal-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu focal-backports main restricted universe multiverse

deb http://security.ubuntu.com/ubuntu/ focal-security main restricted universe multiverse
deb http://archive.canonical.com/ubuntu/ focal partner

REMOVE:

sudo apt-get remove tomboy memtest86+* python3-tinycss* indicator-application  grub2-theme-mint*
sudo apt-get remove xplayer-common* xplayer* xplayer-dbg* xplayer-plugins* gir1.2-xplayer-1.0* gir1.2-xplayer-plparser-1.0* libxplayer-plparser18* libxplayer0*
sudo apt-get remove linux-generic-hwe-18.04-edge* linux-image-generic-hwe-18.04-edge* 
sudo apt-get remove linux-headers-generic-hwe-18.04-edge* linux-hwe-5.4-headers-5.4.0-40* linux-hwe-5.4-headers-5.4.0-42* 

sudo apt-get remove libcec4* libcrossguid0* libdvdread4* libfluidsynth1* libfstrcmp0* libgit2-26*
sudo apt-get remove libgssdp-1.0-3* libgupnp-1.0-4* libhttp-parser2.7.1* libjs-iscroll* libjs-jquery*
sudo apt-get remove liblinear3* liblivemedia62* libmicrodns0* libmicrohttpd12*
sudo apt-get remove libmono-system-runtime-interopservices-runtimeinformation4.0-cil* libnfs11* libp8-platform2*
sudo apt-get remove libpcrecpp0v5* libplacebo4* libprotobuf-lite10* libshairplay0* libtinyxml2.6.2v5* libupnp6*
sudo apt-get remove mint-backgrounds-tricia* mono-jay* python-bluez* python-simplejson* x11vnc-data*

INSTALL:

sudo apt-get install python-tinycss2-common python3-tinycss2 python3-webencodings
sudo apt-get install neofetch ffmpegthumbnailer amd64-microcode intel-microcode celluloid drawing gnote adwaita-icon-theme-full warpinator alsa-topology-conf alsa-ucm-conf mesa-vdpau-drivers mesa-vulkan-drivers cryptsetup-initramfs cryptsetup-run libreoffice-gtk3 gamemode
sudo apt-get install mint-meta-cinnamon xreader xed xviewer pix mintsystem metacity nemo-preview mintdrivers mintupdate mintsources mintinstall

Apt backup in ~/Upgrade-Backup-tricia/APT

sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade

#disable all other repos via sources manager
sudo apt-get update

sudo sed -i 's/bionic/focal/g' /etc/apt/sources.list.d/official-package-repositories.list 
sudo sed -i 's/tricia/ulyana/g' /etc/apt/sources.list.d/official-package-repositories.list 

#remove as above

sudo apt update
sudo apt upgrade
sudo apt upgrade --fix-missing #might be required
sudo apt-get install --fix-broken

#emoval of blocking python2 packages
sudo apt-get purge python-gobject-2 python-dbus python-cairo python-crypto python-setproctitle python-six python-xdg python-xlib 

sudo apt upgrade
sudo apt dist-upgrade

#install as above

restore ppas/apt sources, update to ubuntu focal
sudo apt update
sudo apt upgrade
sudo apt dist-upgrade


sudo /usr/share/ubuntu-system-adjustments/systemd/adjust-grub-title
sudo /usr/lib/linuxmint/mintsystem/mint-adjust.py

As the official Ubuntu Chromium package is a redirect to the snap store version, the Mint team builds and packages it themselves now and it can be installed using:

sudo apt install chromium chromium-codecs-ffmpeg-extra

Add official repository

echo "deb http://repo.mysql.com/apt/ubuntu/ focal mysql-tools" | sudo tee /etc/apt/sources.list.d/mysql.list

Note the key error:

sudo apt update

Insert key in this command:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <key value>

Update and install MySQL

sudo apt update
sudo apt install mysql-workbench-community

sudo ln -s libssh.so.4.8.4 /usr/lib/x86_64-linux-gnu/libssh_threads.so.4

Problem with 3.0.9.2 in Mint Cinnamon 20.1, if you try to close a window while you have a video playing without first stopping the playback, the window will not close and the app hangs, totally. It will not quit, close or any other exit, I have to go into the system monitor and kill the process to get rid of it.

Problem fixed in VLC 3.0.12, but not in repos as of April 2021. Unofficial PPA:

sudo add-apt-repository ppa:savoury1/vlc3
sudo add-apt-repository ppa:savoury1/ffmpeg4
sudo apt update
sudo apt install vlc