Get some 600 grit wet/dry sandpaper at the hardware store. Rub very carefully in the same direction as the lines in your current satin finish. Hopefully, 600 grit will be similar to the final grit size used on your knife. If the original finish is finer, you may need to repolish both sides of the knife to make it look the same.
This is not a terribly difficult thing to do. It will just take a little time and elbow grease. You may want to start out with something like 240 grit, and work your way up to whatever grit size was used to create your final finish. Satin finishes are usually in the 300 to 600 grit range. Just be sure to keep sanding in the same direction as your current finish. This will be either along the length of the blade (handrubbed satin finishes are usually in this direction), or more likely, perpendicular to the edge (original finish applied on a machine driven belt or wheel).
Good luck
