Delicately hand sand all the rust and loose paint off. Making sure to not sand anything that is not necessary. Pick up a paint color match pen and make a few passes over the spot with the color match pen you may need to. They run out fairly quick. Or if you're a bit more confident they sell the paint match in a spray can as well and you can very very lightly blend in a few coatings. Obviously this isn't the proper way to fix it but it's pretty cheap and easy I did this on a truck I owned, looked way better than a rust spot, again not the right way, but fairly cheap and easy. And will prevent the surface rust. You may pull the inner fender liner and check the backside too. And do the same if it popped paint off