back up vs back down

back up and back down both are verbs.

Word	Noun	Adjective	Verb	Adverb
back up	No	No	Yes	No
back down	No	No	Yes	No

As verbs, back up and back down are synonyms defined as:

Other synonyms of back up include back off.

back up (verb)	back down (verb)
establish as valid or genuine	move backwards from a certain position
become or cause to become obstructed	remove oneself from an obligation
make a copy of (a computer file) especially for storage in another place as a security copy
move backwards from a certain position
give moral or psychological support, aid, or courage to