To change the belt on the Simplicity F3500, unplug the vacuum and turn it upside down. There are two tabs (one on each side) behind the beater bar that hold the cover on the vacuum. Press these tabs inward and lift the bottom plate away from the unit.
Then remove the brushroll and the worn or broken belt from the pulley on the side of the brush roll and the drive shaft. Loop the new belt over the driveshaft and the brushroll. Replace the brushroll in the housing and turn it by hand. Make sure that the belt is seated on the driveshaft and pulley with no twists. Also check that all of the rotating parts will move.
Then put the bottom cover back on by aligning the back hinges and then pressing the plate down until both sides click.
For pictures of the cover removal process, look at page 15 in the manual. The direct link is http://www.simplicityvac.com/about-my-simplicity/manuals/FreedomManual_Update0803.pdf
. Most Simplicity vacuum manuals are available at their support site: http://www.simplicityvac.com/about-my-simplicity/manuals/
I hope this helps.