If you have tools, saw the rotten part off, then replace it like other man suggested. If you like me do not have a tool and do not want to buy one, you can do what i tell you. You can manipulate the compound into a rectangual shape and do it a couple of times until the shape matches with the wood. After it dries, it will be as hard as stone.
Youtube it. I followed the instruction on youtube and fixed my rental property. Cost is about $2 because my labor is free.
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