If all the front four screws (2 at the top, 2 on the bottom) of the muffler are taken out, you can usually pry the front muffler half of with a screwdriver. Leverage it under the muffler cover and the front of the case and it will usually come loose.
When new, there is a gasket between the two mufflers halves which sticks the halves together. I usually do not replace this unless the muffler leaks a lot without it.