How to Restore erased areas in Picsart?

Help! This should be easy, but I can't figure out how to un-erase and restore small touch-up areas I previously erased from original photos. I don't mean sequential undo's. But later corrections to restore parts inadvertently erased from a layer. I can erase all I want later - but not restore! Every other photo editing app I've used - Photoshop, Pixelmator, SuperimposeX - have easy methods to switch back and forth between erasing & restoring. Please tell me how I can do that in Ipad Picsart....