are you doing an in-place upgrade on same hardware or a migration to new hardware?
The in-place upgrade should handle everything for you - a migration to different hardware will require manual work (but can still do most of it)
In either case best practice is take copies of BEDB and catalogs before attempting the upgrade
In that case way more complicated and you will probably need to do some research on (or perhaps do a dry / offline run before the date of a more permanent move.)
Firstly - DO NOT run anything but test jobs on the BE 20.x installation before moving the BE 15 data into it as we cannot merge media or job information in this process it is a complete replace.
Also take a copy of the clean BEDB (and the DEK for the BEDB) just after BE 20.x is installed and running (as then you have a way to return to a known state if needed)
Bascially you need to copy the BEDB (with DEK) and catalog folders across between the servers and then use BEMIG to update the database.
You will have to recreate backup target storage and almost certainly retarget jobs to any recreated storage after the migration and if deduplicatioon storage is involved in BE 15 you will need to check the details for moving that (which is another level of complexity over and above the move of the BEDB and catalogs. )