I want to upgrade, how do I select which of my three licenses is upgraded?

I have one that is the oldest but is working fine in a Mac Pro 2013 trash can, I have another that is active in a i7 Mac mini. I wish to upgrade the one that I am currently not using as I want to avoid a startup message on those active computers. I am assuming the "current" version is 18, which will work with the Apple silicon MacBook. I have to ask via forum because apparently they don't even want to assist with a purchase if you have the desktop version past 90 days.

I am retired and shocked by the recent double bump on the home edition price.