how do i reduce the space between an instrument staff and the treble in the grand staff in musescore?

I assume you mean those couple of systems where there is way too much space? That's because there is a chord symbol that was attached to the piano staff for some reason then dragged way way up. MuseScore added extra space to prevent it from overlapping the top staff. Of course, you probably wanted it on the top staff in the first place, so just delete the incorrect one and add it correctly. Maybe this was done because you didn't know how to add chords mid-measure? just press Spasce while adding a chord symbol to move directly to the next beat.