Here are some ways...
1. Give the unit a tome of health.
2. Abuse the maximum HP bonus item abilities's incorrect level up and removal behaviour to give extra hitpoints.
3. Abuse a unit to hero and hero to unit converion via a morph ability to give extra hitpoints.
4. Use an set of item hitpoint abilities with binary values to create a system to change maximum hitpoints within a certain range.
5. Make the unit a hero and add strength.
6. Use an upgrade.
WARNING
1. Tomes will leak unless specifically removed after use via the appropiate native.
2. Will bug if the unit changes type giving ever more hitpoints every change.
3. Is highly unstable and may alter other stats, use at your own risk.
4. Can add signifficant load time to your map if large ranges are required.
5. Can also increase hitpoint regeneration and even damage.
6. Is permanent and can not be undone.
I advise method 4. Most reliable and with 20 odd abilities it can cover a huge range (more than most well made maps should ever use).