What exactly have you tried and what was the result?

Note that normally you need to do the following:

Put your backpack, which is not ruined, on the ground.
Put the fixing tool (duct tape, leather kit, NOT sewing kit) to your hands.

Switch off the inventory view.

Move your focus point / crosshair to point at the backpack on the ground. (An option to fix will appear)

Hold the left mouse button.