Health is increased in Assassin's Creed Revelations by purchasing armor from one of the many Blacksmiths.
1. Open your map by pressing select (PS3) / back (Xbox 360) and locate the nearest Blacksmith.
2. Navigate over to the Blacksmith and press O (PS3) / B (Xbox 360) to shop.
3. Select Armor
4. Select one of the armor items that you do not already own and press X (PS3) / A (Xbox 360)
Notice the health increase on the right side
5. Your health will be increased in the top left corner.