Assuming you are referring to a roll-up tonneau cover:
Most roll-up tonneau covers have some kind of adjustment mechanism to tighten or loosen the cover. This is usually done with a simple ratchet system. To adjust the tension, first locate the two side rails that run along the inside of your truck bed. There should be a small knob or handle on each rail. These are used to loosen and tighten the cover.
To loosen the cover, turn both knobs counterclockwise until they stop. This will allow you to pull up on the cover and make any necessary adjustments. To retighten the cover, reverse the process by turning both knobs clockwise until they stop.