Concerning angels' wills, good and bad, St Thomas writes (Sum. Theol. I, q. 64, art. 2):
"Appetitive power is proportioned to apprehensive. Now an angel's apprehension differs from a man's in this, that the angel apprehends a thing at a glance, by one fixed intuition; man by a course of reasoning, inclining him to opposite conclusions. Hence man's will adheres to an object unsteadily, but an angel's fixedly and immovably." See above, Chap. LV, #4.
Thus naturally there is no repentance for fallen angels.
Of God and His Creatures: 4.95