SPK for Synology is available here:
https://synocommunity.com/package/radarr

Synology DS414 uses armv7l CPU MARVELL Armada XP MV78230

Manual update when auto-update fails:

sudo synopkgctl stop radarr

cd /var/packages/radarr/target/share/
sudo mv Radarr Radarr-old
sudo mv /tmp/radarr_update/Radarr .
sudo chown -R sc-radarr:radarr Radarr

sudo synopkgctl start radarr

Issue updating Radarr with error

[v3.1.0.4893] System.UnauthorizedAccessException: Access to the path '/proc/19002/maps' is denied.

can be solved by stopping Sonarr first, applying the Radarr update and then re-starting Sonarr.