Is it easier with version 4.5?
it should be the same. You can try on your existing image - opening the RGUI for the game you'd like to change the inputs and change the controls until you're ok, then save a game override.
Where can I get a reputable installation file? or can I do that via the retropy menu? I'm a bit afraid that nothing will work after that.
The only image we support is the one that you can download from retropie.org.uk/download.