Cast to another actor -> What exactly does Object refer to

Hey, I have been trying to learn UE4 the last few days.

One thing I can’t get my head around however is how exactly to “cast to” other actors, irrelevant of the actors type.

I have seen many examples that use very specific blueprints like your player or gamemode, but none of that really help me in my situation. In my current project I am trying to cast to an actor I am using as ground for my level. On this actor is a TextRender component, whose text I want to update from the blueprint I am currently in.

Could someone explain to me how to determine what object I need to use, preferably with my current example in mind?