Bolster Dead

Choose three undead under your control or the control of an ally. Such undead gain +2 AC,  and their hit point maximum increases by 10. Additionally, you can choose one ability score above 10 to increase by 2; if you choose an ability score below 10, it increases to 10. The undead has proficiency in the ability score you chose to increase. 
If this spell is cast on the same undead again, it loses the effects of the previous casting of the spell, instead gaining the effects of the new casting.

At Higher Levels. When cast using a spell slot of 4th level or higher the maximum hit points added by this spell increase by 10 per slot level above 3rd.