I have little dents on the back of my phone

You can replace your back cover, but it will be expensive. You must completely disassemble your iPhone, buy a new back cover and then reseat everything. I think it is not worth doing. You have two options:

buy replacement back cover from somewhere (eBay has bare back cover and half assembled - I would go with half assembled because it has buttons, dock connector and jack already assembled, so you just swap some small parts, main board, camera, display and battery and you are good to go)

or buy iCloud locked iPhone with great condition for parts and replace the logic board and button (it is more expensive than the first option, but you will have parts already with you when something goes wrong with your phone).

It will void you warranty, so be sure to not mess something up or you will be on your own. Apple will not replace or repair iPhone that has been tampered with.