How is the V-belt tension adjusted (Hoover Model

How is the V-belt tension adjusted (Hoover Model U6439-900)or how do I know if the tension is correct? There seems to be a lot of "play" in the belt and I'm wondering if that is the reason the brush roll only turns as the wheels turn, regardless of the carpet height setting. And yes, the brush roll is set for carpet, not hard floor. Or is the cause of the problem something other than the V-belt? A new flat belt was installed when the machine was professionally serviced so I assume (?) that was adjusted correctly.