Your Intelligence score is 19 while you wear this headband. It has no effect on you if your Intelligence is 19 or higher without it.
Notes: Set: Intelligence Score, Buff, Headwear
Could I raise it after putting it on with an ability point increase?
It sets your Int to 19. ASI don't matter. if you intellegence is above 19, then then it does nothing, if its below 19, its 19 until you take it off
Question was if I wear it, and then use an asi, does it go to 20, or is it 19 regardless (until removed)?
It stays at 19 until removed. For example; you put in on a character with 15 Int, their score goes up to 19. adding an asi to put it up would simply make their Int 16, but the 19 would still override until it is taken off.