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Canvas can be cleaned somewhat with a bleach and detergent solution, using a soft brush to work it into the canvas and allowing 30 minutes of so of soak time. However, the canvas will have to be re-waterproofed thereafter.
The entire canvas can be replaced. If the canvas needs other help, this may be an option. I had great luck with Anchor Tent and Awning in Evansville, IN, but that was quite a few years ago. My new one fit better than the original!