Changing Playstation Discs In Retroarch (READ DESCRIPTION)

When prompted to change discs, locate the Command menu in the top left corner of the emulator window. Select Command, go down to Disc Options and select Disc Eject Toggle.
Then press the F1 key to open the Retroarch quick menu. From here, go down and select Disc Control, then Current Disc Index (to change the disc).
After you select your disc, select the insert disc option. Then press START on your controller and you're done.
Lastly, if the disc you want isn't in the Current Disc Index. Make sure the virtual tray is closed, then use Disc Control again in the quick menu (F1) and select Load New Disc.