2. 'Back off' means reverse off a particular object. For example, "Back off that grass verge right now!"
3. 'Back up' means reverse in a more general sense. For example, "I want you to back up until I give you the signal to stop."
Note that all three of these verbs have other meanings than reversing:
1. "I was forced to back out of (renege on) the agreement."
2. "If you don't back off immediately (stop behaving aggressively), I'm going to hit you."
3. "I expected you to back me up (support my position)."