Reverse it up some car ramps ( not easy with no power ) or otherwise get it about 1 foot off the ground.
Make sure it is very stable because if it falls while you are under there you become lawn fertilizer.
The belt is a strait triangle from the engine pulley around both the drive pulleys .
They are a long way up and not easy to get at,
The tension pulley pushes in on the exit side of the belt
Take the spring off
Generally there is a square hole in the tesioning arm for a ratchet or breaker bar.