Below is how you install Deluxe and Premium Deluxe after the Standard version has been downloaded:
Go to menu / Profile / Content manager / and selected “toggle all” now the button says “Download Selected” so that you can install the additional goodness of Deluxe (?GB) and Premium Deluxe (3,79GB).
Note, that you might have to enter, wait a bit and re-enter a couple of times into the Content Manager and packages will show up eventually. This has been the case for a MS Store purchase. For Steam purchase the packages might be available later but as soon as possible.
I tried Scuby’s method. A couple of airports were ready for updating but no sign of the 5 extra aircraft that I paid for (p. Deluxe) and never did get.