First of all, it depends on how severe the damage is. If there is just some light scraping or some scratches, they can usually be polished out of the screen using a fine grit polish.
If the damage is more severe, if there are cracks going all the way through the screen, it will most likely need to be replaced. The process for this is as follows. You will need an iSlack or similar wedge device, and a replacement phone screen.
1. Remove the front glass by unscrewing it at the bottom. iPhones use Pentalobe P5 screws, so you will need to buy the right screw driver.
2. (optional) Heat up the front of the phone, to soften the glue holding the screen to the phone. This will make the following steps easier.
3. Use an iSlack or similar device to raise up the side of the front glass. Do this first on the left side, as most phones have sensitive ribbon cables on the right side.
4. Then, go all around the edge of the phone, raising it up on all sides. Be extra careful on the right side not to break the ribbon cables.
5. Open the phone like a book, from left to right. This way, you will be able to gently unclip the ribbon cables, and then lift the entire phone screen off.
6. Connect the ribbon cables of the replacement screen. Then close it like a book, from right to left.
7. Firmly press the screen in all the way round the edges, until you hear a click. Press the screen in carefully, being sure not to bend or break the phone.
That's it! You have replaced your very own phone screen, and saved money on a new one. If you found this answer helpful, I would really appreciate an upvote. Thank you for reading this whole answer, and have a good day!