Empty the backpack completely
Inspect the tear and the surrounding fabric
Trim any loose threads
Align the strap back to its original position
Sew the strap with heavy-duty thread using a strong box stitch or X-stitch
Reinforce the repair with a patch of durable fabric on the inside or outside
Use a sewing awl or heavy-duty needle if the material is thick
Apply fabric glue only as a temporary fix
Add a second layer of stitching around the repaired area
Test the strap gently before loading the backpack
Replace the strap if the damage is too severe