First, removing the old snap simply requires using a new, sharp drill bit that is smaller than the hole drilled in the body. This is important as you will tend to wallow out the hole in the body and new pop rivets won't grip anymore. Had this happen to more than a few of mine and had to back all the pop rivets with washers. Not an easy task in all areas, but doable. The last thing you want is a snap that is not snugged down tightly. Also, might I suggest that you purchase the stiffer Dot brand female component of the snap set for your top and cover. These things don't hold well after a while. NEVER use the cheap snaps from the hobby store. They are plated steel and will RUST. Stick with Dot brand. A good upholstery shop should stock them. May have to grip the male and the female snap component with pliers to hold them steady when drilling. Just as easy to take the old snaps out of the top to freshen them with new ones. May have to back them with a small piece of old top material glued in place.
Good luck
Bill