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I am sorry if this question seems obvious, I just cannot find the answer anywhere.
HP increase every level with the hit dice roll and the constitution bonus is added every level. What happens when the constitution bonus of the player increases because they found an item that increases constitution?
As an example, a character level 5 with 16 constitution finds at level 5 an item that increases cons to 18. Does the player get in total +4 hit point for each level or only from the 5th on?
If doesn't matter why a character's CON changes (increase or decrease), the character's maximum HP is adjusted to match their new CON bonus. This is explicitly in the basic rules for Constitution.
If a character’s Constitution score changes enough to alter his or her Constitution modifier, the character’s hit points also increase or decrease accordingly.
This rule is why Rage increases a Barbarian's current and maximum hit points. It is why Damage or Drain to Constitution can reduce a character's current and maximum HP. This is why magical items that increase CON are desired by every character of every class.
Bonus Answer
The Damage a character has taken remains constant when HP changes due to CON changes. This is why Barbarians sometimes fall over dead when Rage ends - reverting to their normal Constitution reduces their current and maximum HP, but the HP damage remains; thus a Barbarian with 30 HP can suddenly drop to -50 HP and be very, very dead.
This feature is also why CON damage and drain are so dangerous. Characters have just as much HP damage but have fewer HP to absorb that damage.
