Leather cleaners are available at most fine furniture stores, but my experience hasnít been the greatest with these cleaners.
I always seem to resort to an enzyme cleaner called Pink Solution. Apply it with a spray bottle (trigger type) and dampen the surface to the point the leather is wet.
Massage with your finger tips until you work up a lather. Add clear water with another trigger bottle and blot with a soft white cloth. Repeat until clean. Allow to dry. Apply a leather conditioner to entire chair back and arms.
As with any cleaner used on leather, be sure to try the Pink Solution on a test spot (maybe on the bottom or the back of the sofa) before you apply it the obvious areas on the front of the sofa. You will want to make sure that the leather dye is not affected by the cleaner.