First of all, an 'over the top swing' is referring to the downswing path the golf club travels on. That path is 'over the top' of the correct swing plane or path that the club should be swinging down. This path causes the club to come into the golf ball from too steep of an angle on the downswing and often results in pulls and/or slices.
Here are a few tips on how to go about fixing this issue. Assuming you have made a good backswing and are in a good position at the top of the swing, then you want to simply swing the club down the same path or plane on the downswing. Golfers who come over the top are essentially creating a loop in their swing and the club will be forced to swing to the left of the target just after impact either causing a pull or a slice depending on how open or closed the club face is.
Simply focusing on swinging the club down the same path that you swung it up on in the backswing can help. Another tip is to try and keep your back facing your target a little longer on your downswing. You want to feel that your arms are swinging the club down but the upper body stays pretty stable as this is happening in the first part of the downswing. Golfers who move their upper body or their shoulders incorrectly from the top of the golf swing end up re-routing their club and can swing over the top.
As you start the downswing, you need to feel that the club is swinging more from the inside and on a better path. This way, the club will travel on a good path and you'll be able to deliver the club through impact correctly resulting in straighter golf shots.