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 init=1.51 iproute2=4.15.0-2ubuntu1.1 kodi=2:17.6+dfsg1-1ubuntu1 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:

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

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

INSTALL:

apt-get install python-tinycss2-common python3-tinycss2 python3-webencodings
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
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 '/bionic/focal/g' /etc/apt/sources.list.d/official-package-repositories.list 
sudo sed -i '/trisha/ulyana/g' /etc/apt/sources.list.d/official-package-repositories.list 

#remove as above

sudo apt update
sudo apt upgrade
sudo apt-get install --fix-broken

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/ eoan 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