Repairing a sill plate is something that any homeowner with a basic set of carpentry experience and skills can do himself.
Step 1 - Remove the Old Sill
To remove the old sill, pry it from the floor with a hammer and chisel. Do it carefully, so as not to damage the subfloor. If any flooring is damaged in the process, it needs to be repaired or replaced before continuing.
Step 2 - Prepare for the New Sill
Before you install the new sill, measure it to make sure it fits. You may need trim it to fit your doorway. Before installing the sill, apply some silicone sealant to the area where the sill meets the interior flooring. That will keep the connection safe from debris and water getting in.
Step 3 - Install the New Sill
To install the new sill plate, place foam insulation underneath it and make sure that all gaps are sealed. When the foam sets, it will act as an adhesive in itself. You will not need any nails. Use wooden shims to apply downward pressure on the plate until the foam sets. Remove them after several hours. Finally, apply a coat of varnish and a coat of moisture resistant sealant if the sill is from an exterior door.