Well there are a few options.
The easiest would be like eXi suggested to use the “Get all Actors of Class” node and select your Blueprint. You could also pass it down via other blueprints or use an interface in your UMG blueprint which takes your object as input and sets it to your variable.
However I personally would suggest not updating the score in a separate actor blueprint but rather doing it in the GameMode blueprint. It sounds like a very GameMode type of thing to do ;D
And a reference to that is easily obtained. Just use “GetGamemode”.