Make sure that you know the location of the relevant Photoshop executable file. (I'd normally expect that to be photoshop.exe but, if you've got two different versions of Photoshop it might be something else. Alternatively you could just have two occurrences of photoshop.exe, in different locations).
Right-click on a file which you want to open with the relevant version of Photoshop. Select 'Open with' and then click on 'Choose Default Program'. (It doesn't matter if you don't actually want to set that version of Photoshop as the default program for opening that type of file. Do it anyway).
You should then see:
(a) a pre-ticked box, setting your choice of program as the default option ; and
(b) the 'Browse' button which you need to locate the .exe file.
If you don't want to set the program you're looking for as the default one, click to remove the tick. Then click 'Browse', navigate to the location of the .exe file and double-click on it.
The file should then open in the required program but, more importantly, Windows should see the program you chose as an option to offered from 'Open with' in future.