The first time ours tore, I tried to fix it. But the sun had done it's damage and the canvas was basically "rotten". Everywhere I tried to pull it together it tore more.
Broke down and ordered a replacement cover from the company - could have almost bought a new gazebo for what I paid! the second time I just let it go. My climbing Cecil Brunner Roses had reached the top and had almost covered it. By the next year the arbor top was completely covered.
If you're not into plants hopefully someone will have a better answer for you on how to fix or replace the cover. Good luck
I use Gorilla tape on both sides of the tear. It comes in clear, as well as white and black.
This might help. https://www.hunker.com/13415451/the-best-way-to-repair-a-patio-canvas-awning
Hello I patched mine with a piece of canvas and fabric glue then faux repainted the striping to disguise the repair.
Where exactly is it torn? If it is in the body of the fabric (in the middle) then as Debbie Dragonfly referred to, the fabric may just be disintegrated due to exposure to sun and weather.
If it is torn at the seams, or at where it attaches to the frame, then it is likely due to tension and abrasion. These kinds of rips are more repairable.
If you posted a picture of the tearing, we could instantly see what the tear looks like, and then would be more able to advise you.