Start by loosening, but not removing, all of the nuts and bolts that attach the exhaust system to the motorcycle. If you're only replacing the muffler with a slip-on, there is probably no need to loosen the head pipe. Once everything is slack, remove the bolts holding each component, then remove the components, starting with the muffler. This may be easier said than done, particularly if the bike has some miles on it and heat and corrosion have done their dirty work. If the muffler is just stuck, spray some sort of rust-busting lubricant, such as WD-40, CRC-556 or Liquid Wrench, into the muffler joint, let it stand for a few minutes and then try twisting the muffler slightly as you pull backward. If it simply won't budge, place a block of wood against the mounting bracket and give it a few good raps with a hammer. If you have a helper handy, have him (or her) pull and twist the muffler as you pound on it. In most cases this method will free up the most recalcitrant muffler, though it may not look like much when you're done.