My guess is you're having trouble getting under the mower to relieve the tension off the pulley with one hand and take the belt off with the other. If that's the case then take a rope and run it through the tensioner arm so that you can keep one hand out from under the mower to pull back the pulley with ease then use your other hand to take the the belt off of the other pulleys. I hope this might help.
Tie the top off at the engine end , loop the rope around the tensioner pulley bolt and pull on the other end that way you get a 2:1 advantage over the spring.
I use a length of No 8 Full hard fence wire ( in plentiful supply around here ) hook on one end other wound twice around a conveniently sized rod. old shovel handle etc etc,
Make it long enough to reach from the pulley to the back of the machine so you can brace your feet against some thing handy then pull on the wire with one hand while you slip the belt over the engine pully with the other.
A similar tool ( different length obviously ) can be made to remove & replace the tensioning spring, deck lift spring or any other spring for that mater.