So I have this angelfish that I brought and when I got home I realised that it has a bent fin. And I don't really want to return it and I don't know if there is a way to fix it. And sometimes it is not facing down like the picture.
Sometimes angelfish have birth defects. I used to have an angelfish with one of his sides caved in, and he is still alive! Just as long as the birth defect isn't affecting its swimming, then he/she will be fine.
Pretty common for veil-tail type angels IMO. My koi angel's finnage was just like that at that age. Not the best pic, but here's a recent pic. The koi is 3-4yrs old and the silver is maybe 1.5yrs old: Here's a younger pic of the same angel:
Angelfish fins go like that for two reasons poor genetics or water quality not good enough while it was growing up. Once they are bent they stay that way. They still make good pets, but I wouldn't raise any fry from a fish that looks like that.