freshen vs freshen up

freshen and freshen up both are verbs.

Word	Noun	Adjective	Verb	Adverb
freshen	No	No	Yes	No
freshen up	No	No	Yes	No

As verbs, freshen and freshen up are synonyms defined as:

Other synonyms of freshen include refresh, refreshen.

freshen (verb)	freshen up (verb)
make fresh again	become or make oneself fresh again
become or make oneself fresh again	make brighter and prettier
make (to feel) fresh