There are times when canvases become torn (or I get a great deal on one that is) and so I like to repair them in a way that is archival so it won't eventually eat through the canvas, stain or come apart.
an old paintbrush
a pair of scissors.
1. First, I cut a patch a little larger than the tear. I tend to make it about an inch wider and longer to ensure that it will adhere well.
2. Using my paintbrush I add a fairly thick layer of gel media to the back of the canvas, place the patch on, and repeat. I smooth out the torn edges on the opposite side and using my fingers hold it firmly while I smooth out the gel media on the back.
3. I turn the canvas to face me and add a fairly thick layer of gel media over the tear and smooth that out well.
4. Once the gel media is dry, though you can still see the torn area, when I feel it with my fingers it is fairly smooth. And, in any case, paint will cover it.