Brake set to give max slack in belt.
Remove belt from clutch pulleys
Drop the drive pulley and slip belt off the pulley but not the shaft ( sits between the drive & deck pulley ) on manual PTO or right off on electric PTO.
Then the rear pulley.
Slide the belt off one edge of the pulley and between the fan blades while rotating the pulley.
On manual systems remove the belt from the front stack.
Reverse to fit the new one.