Since I am a complete noob when it comes to this sort of thing, I wanted to make a guide that's easy to understand, and should therefore be easy to follow for other newcomers, etc.
Okay, step one:
To see if it worked, go to File> Navigate and find your backsprite again, like this.
That's how to do one, but if you notice, there are 5 different backsprites.
For the 3rd, 4th and 5th backsprite frames, repeat steps 12-14 with the respective sprites.
If it all works out properly, you should end up with your new backsprite in-game
I hope this helped someone, I tried to make it as simple as possible.