To retouch something is to improve it by making small changes. You might retouch your photo to make your teeth look whiter before you post it on social media.
When you make edits or alterations to a photograph, a painting, or another artwork, you retouch them: "It's clear the artist decided to go back and retouch these early etchings." You might also retouch your friend's makeup or hairdo before the prom and then use computer software to digitally retouch the pictures you take of her. The French root, retoucher, means "touch again with a view to improving."