I'd like to give my Players additional modifiers according to their backstories etc. How do I do this?

Thank you!

You mean, like, +1 Str, that kinda thing?

Modifier: Bonus->Subtype: [Ability]->Fixed Value: X

There’s no way currently to add modifiers to backgrounds however. You would have to add that to a feat and give them that directly on their character sheets.

Alternatively you could adjust their character sheets directly.

How can I add it on the character sheet directly? When I tried the override it seemed to be for the entire score? 

Thanks for your help! 

Any bonus you give to an Ability will always be applied to the Ability Score, not just the Ability Modifier.