There are two ways to fix it. If the wood is not too far gone you use an epxoy product to strengthen the wood. I believe the product is called Rid Rot or something close, check with a local boating store and ask them for something to repair a wooden boat. If it gone too far you'll have to cut out the ould wood and replace it with new plywood. Be sure to find the source of the leak Vent, Seam, window or whatever and fix it so the problem does not re-occur
75 Argosy 26'
96 Ford F-250 PowerStroke