There is no way to repair large areas of rotten wood; it will simply have to be replaced. If you can see the nails that are holding the siding to the house, pull them out or drive them in with a nail set. Sometimes it is difficult to see all the nails if the siding is covered with several coats of paint. If this is the case, pull up the siding with a pry bar or shims until the piece pulls free. It is important to replace the piece of siding with a piece of the same type of wood. Take the rotten piece to a hardware store or your local lumberyard if you are not sure what type it is. Prime the new wood on all sides with a sealer before installation. It can then be cut, put into place, and nailed in.