If the canvas is torn, you could stitch it or use mending tape.
Here's a video that might help - https://www.youtube.com/watch?v=AQnPp9B0KUM
Hi Elizabeth, her's a great video that will help you fix the canvas - https://www.youtube.com/watch?v=mkvUn1RX1qE
I'm been wondering this myself! I saw some new ones you can purchase on Amazon.
If it's just a small tear, you could use a sewing awl (often used for leather, but works well on thick or heavy fabrics). This will give you the punching power to get the needle through the canvas.
You can try to patch it or remove it, buy new material and use the old one for a pattern to make a new one.
Hello neighbor of mine has had a problem with the bottom seat part of the canvas glider swing. They replaced it with wood
Replace the whole timber for a strong job!