There is usually a small piece of metal on the other side of the fabric. This has slots in , through which tabs on the bar or hook pass then simply bemt over to secure in place.
You may be able to lever the tabs up (but be careful not to slip and jam whaever you are using to kever into your hand). Alternatively a small pair of instrument wirecutters can be useful to get under a tab and lever it up.
Acp, July 2017