You cannot "upgrade" between releases, in the sense that there is no program that will take your r2019b matlab and modify some libraries and sections in-place transforming it into R2020a instead. It is not like renovating a room where the old room is gone afterwards.
Instead all there is, is also installing R2020a as well as your r2019b.
To do this you need a license for R2020a and whatever toolboxes you need. The installation process will connect to Mathworks and check your license, and will offer to download and install the R2020a version of whatever is covered by your license.
Afterwards both versions would be installed.
You can then test your code under the new version and if it all works you can remove the older version.