(けんきょ) This is a word you use about others. “She’s got a very modest attitude.”
(けんそん) To downplay your own skills in front of others. You don’t ever say “I 謙遜-ed.” or “She’s really 謙遜-ish.” Normally it’s only used like:“Please don’t kenson. We both know you’re rad.” or “He declined the offer out of modesty.”