Hello nymnym999,
You did not specify which Fusion amp you have but if the insides looks like the Fusion PP-AM60020 image below, you'll have to "desolder" the transistor.
First remove the heat sink from the transistor case. You'll need a controlled heat soldering station, a desoldering vacuum tool commonly referred to as a "solder ******", and some desoldering braid. Be careful with the heat around other components.
After removing the transistor, use the desoldering braid to clean off any remaining solder from around and in the circuit board mounting holes. Position the new transistor and solder it to the board. Put a thin film of thermal compound on the heat sink and remount it to the new transistor. Use just enough of the compound to insure an air-free join between the transistor case and the heat sink. Reassemble the case and you're finished.
Hope this helps.