How do I change the belt in a Hoover U4262-930

You got the second hardest part done; the hardest part is what you're asking about.
To get the new belt onto the drive shaft I used a long screwdriver for leverage and wrapped the end close to the shaft and with some elbow grease, got it to "slip" on to the shaft. It took a couple of tries, but once I overcame the fear of the belt breaking (because it has to stretch big time), I managed to get it one. Of course, the other end of the belt has to be wrapped around the roller first.
I found a link for a tech manual for this model, but it didn't help much....
good luck.