Cleaning a moldy sofa or chair is challenging—unless the cloth is removable, in which case, simply follow the fabric instructions above.
First, inspect the area to see just how far the mold has grown. If your upholstery is synthetic, mold growth could indicate that the framing itself is moldy (and you might need to replace your furniture). If it is cotton, wool, or another natural material, odds are that you can spot-clean the area. Take a cleaning brush and scrub the area with soap and water to remove visible mold, then blot the area with a solution containing equal parts hydrogen peroxide and white vinegar. Use a towel to pat dry, and then let the upholstery thoroughly air-dry.
If the mold covers a large surface area of your upholstered furniture, there are a few extra steps you’ll likely need to take. First, bring the furniture outside to air out in direct sunlight, like you do with leather. The UV rays will kill the fungus before you remove the mold and mildew stains. Next, vacuum the upholstery using a brush attachment (making sure to wear your N-95 mask and long sleeves to avoid contact with any spores); empty the vacuum cleaner outside to keep mold from reentering the air in your home.
Then, mix water and a mild detergent until it suds up, then scrub the suds into the spot with a scrubbing brush. Wipe away the excess, then let it dry again in direct sunlight to kill any remaining bacteria. If you see some spots of mold left over, follow the above spot treatment method again to finish the job.