Set the tension to the lowest position where it is still attached, and then using a flat bladed screwdriver of about 20" long, pry the new belt over and onto the pulley. Once installed it will just need the tension adjusted.
I purchased a aftermarket belt to replace the original belt on my LandPride rear finish mower, and had to do the same just to get it on. Once on, I adjusted the belt for proper tension about 1/4" when pushing with your thumb between two pulleys. That belt has no signs of wear and if it lasts half as long as a LandPride OEM belt, I will be happy. Changing a belt is fairly easy, and a lot easier than handing over more than $200 for what you can purchase for half or even one quarter for aftermarket.
Just keep in mind that it is rust on the pulleys that destroys a belt. Keep your pulleys clean and rust free and they will last you a long time. When winter comes, remove the belt and put it away. Spray the pulley surfaces with WD40 to keep them from rusting, adn in the spring before you put the belt back on, wipe the pulleys with some spray brake cleaner and a clean cloth to remove the winter dirt. Sure it is going to take some effort on your side, but unless you are extremely wealth, you will save some of your hard earned $$$$.