Hitting the ball with the bottom of the club face can result from several different swing faults
Changing your grip is uncomfortable so the best place to work on it is at home, holding the club a few minutes at a time until it feels comfortable enough to swing with
Focus on your left wrist at the top of the backswing and start of the downswing to get to impact properly, many players have a cupped left wrist which changes the angle of the clubface, this causes an open clubface and results in casting and manipulation during the last portion of the downswing