Combine equal parts of vinegar and water in a spray bottle. Spray the entire couch with the solution and allow it to sit overnight.
Wipe the couch with a clean cloth and warm water. Start from top to bottom and remove all of the vinegar solution. Pat dry the couch with a towel.
Apply linseed oil to the entire couch using a clean cloth. Rub the oil into the leather in a circular motion, working from top to bottom.
Wipe away excess linseed oil using a clean cloth.