mend (one's) ways
To start behaving in a different, usually preferable, way. After I got in yet another fight at school, the headmaster told me that I had to mend my ways or else I'd be expelled. No matter how old you are, there is still time to mend your ways.
mend one's ways
Fig. to improve one's behavior. John used to be very wild, but he's mended his ways. You'll have to mend your ways if you go out with Mary. She hates people to be late.