It has been a long time since I was using mint but the last time I did a upgrade I followed this stack answer. Not sure it still works but might be worth a shot.
https://unix.stackexchange.com/questions/327808/unable-to-locate-package-mintupgrade
It has been a long time since I was using mint but the last time I did a upgrade I followed this stack answer. Not sure it still works but might be worth a shot.
https://unix.stackexchange.com/questions/327808/unable-to-locate-package-mintupgrade