The best option to repair your cover is to cut up the storage bag that the cover comes in. This is made from the same material as the cover itself. Then you stick the new piece to the inside of the cover and fix it with PVC glue or Bison Vinyl PVC Soft Plastics Repair Glue.
Once you have attached the new piece to the cover, make sure you stick it between, for example, a table and heavy books or a heavy box.
The quicker and cheaper option is to cover the tear or damage with a good quality duct tape. Please note that this is less effective than sticking a new piece of cover over it.