GMC Sierra Denali 2006 Owner's Manual page 110

3. Remove the two bows by gently pushing them
against the spring-loaded end. The two bows
are labeled front and rear to remind you of the order
in which to reinstall them.
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To clean the tonneau cover, use mild soap and water.
If necessary, use a soft bristle brush to remove dirt
trapped in the grain of the material.
4. To add cargo to the
box, remove all bows
and the back rail
and stow them safely.