I need to upgrade to 19.3 or 20.1 by April anyway, so started 18.3 to 19.0 in place upgrade
You do this,
https://www.tecmint.com/upgrade-to-linux-mint-19/
then 19.0 -> 19.3 and 19.3 to 20.1 (may need 20.0 first). After the 1st upgrade to 19.0 it's GUI based.
Everything is backed up and also I have an older inferior laptop already with 20.1, Calibre 5.11 and all my data files, about 300 G Byte