It's easy to remove the screw, but hard to explain it in writing.
You can use something like a Dremel tool to grind the little ramps on the one way screw and use a screwdriver with a good sharp square blade to turn it counterclockwise.
The method I use is quick and easy. Open the unloaded action and clamp it in a well padded vise. Push the safety to the fire position. Using a 1/8" drift punch, tap the back of the slot of the screw counterclockwise. By tapping lightly, you can walk the screw around until you can grab it with pliers. If you are careful, the open bolt will hold the safety lug in place while you install the new safety button. Be careful not to lose the little ball.
Vang Comp makes a very nice oversize safety button for Mossbergs. It comes with a socket head screw and lock washer. Wolf makes a steel replacement button that costs about $7.