If you have a witch who is at level eight or above in their magic skill, they can blast a Sunlight Charm at a werewolf in their wolf form, which will change them back and force them to finally have a birthday (if they've been stuck at the end of their life stage, anyways). That's the most guaranteed way to do it.
Sometimes werewolves will just change back if you keep an eye on them, i.e. invite them over all the time, keep them on an active lot, etc. It's not as guaranteed as the Sunlight Charm, but it's something to consider if you don't have a skilled-enough witch.