You download the desired PAT file from synology site and browse to control panel -> updates in the DSM GUI itself and thats where it asks for the PAT file (under manual install).
I waited for the countdown timer for reboot to finish and then booted in TCRP to complete the rest.
Obviously take backups of existing USB configuration / your data sitting on DSM HDD's if you can before attempting any of this 😃 otherwise, update at your own risk.