if you press select + Y (or X, not sure), you should get the retroarch menu. within that you can set a couple of options that increase the resolution of PSX games – they’re in the ‘core options’ menu, which is behind the ‘settings’ or ‘quick menu’ i think. they’re called ‘enhanced resolution’ or something like that.

i should say that i get performance/sound issues with these settings, but it seems to depend on the game.