Hi John, its an easy enough job, only bolts hold the fan to the heater housing, however you have to remove the heater housing to get at it.
remove heater hoses, remove control cables, undo 5 bolts holding heater to bulkhead and you should be able to wiggle it out.
while its out it may be a good idea to drill out the rivets around the top of the heater and check the matrix, i have packed the sides of mine with dense foam to stop the air getting around the sides of the matrix.
the original foam had broken up and allowed the matrix to rattle about.
a tip for you is to drill a small drain hold in the bottom of the fan chamber to allow any water to drain out.