1. Save first as this can make the follower hostile sometimes
2. Equip and ready the Wabbajack
3. Dismiss the follower
4. Immediately hit them with the Wabbajack until the follower changes into something.
5. When they return to normal, they will be leveled to your current level, within their range. Some followers have a maximum level.
6. Rehire them
I forget if you have to hit them again with the Wabbajack to make them come back or if they pop back to normal after a moment.