I'll have to settle for MC19 and the loss of money to a non upgrade.
I suggest that you wait until a couple of weeks after MC21 is released, and then ask for help again when the process is fresh in all our minds. That will save you going through two upgrade processes.
It isn't a difficult process, and if you have tried to install MC20, then you probably have both MC19 and MC20 on your PC right now, since they will install by default into different directories. I suspect that you are trying to start MC by clicking on the old MC19 icon, rather than the new MC20 icon. They look the same, but will be in different places.
Anyway, wait if you like, and then ask for specific help.