Also, if you back a car up, that means you slowly drive it in reverse, usually into a certain position.

to prove that something is true. By example He hasn’t backed his statement up with facts.

Phrasal verbs can mean different things in different contexts. On a computer, if you back something up, it can mean that you make second copy just in case something goes wrong with the original copy.

All three answers below are correct, and there are probably even more. You need to give/have some context to know which one it is.