The chief justice has significant influence in the selection of cases for review, presides when oral arguments are held, and leads the discussion of cases among the justices. Additionally, when the Court renders an opinion, the chief justice, if in the majority, chooses who writes the Court's opinion. -wikipedia

Chief Justice is also responsible for a few other things, a lot of which are ceremonial. It's not an all powerful role but it is influential.

And Trump can't change the Chief Justice. Not unless there is a vacancy. So if Roberts is impeached, retires, or passes away, only then could Trump appoint a new chief justice.