How do I ad 1 charisma on my character's ability score?

My character got an item that gives him 1 point on charisma, but how do I had that. I tried to change that on abilities where you add the points when making a character, but when I go back to my character sheet it just says the same number that I had. 

If you click on your charisma score in the main character sheet there should be a place to add a modifier.

What is the item? If it is created correctly, added to inventory, and equipped, the bonus should automatically be applied to your character. Changing your character's base CHA isn't really the best way to do it, because the bonus comes from the item, and items can be lost/destroyed/traded/stolen/etc.

If it's a homebrew item, check the modifiers and make sure it has a bonus CHA mod of +1. If you're unsure if it's added correctly, you can create a homebrew copy of +1 magic item and look at how the modifiers are set up for reference.

Note: if your cha is already at 20 it won't increase unless you raise the ability score maximum or use the stacking bonus modifier.