The Collaborative International Dictionary of English v.0.48:

touch up \touch" up`\, n.
   To make minor improvements in, especially in the appearance.
   [PJC]

WordNet (r) 3.0 (2006):

touch up
    v 1: alter so as to produce a more desirable appearance; "This
         photograph has been retouched!" [syn: touch up,
         retouch]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "touch up":
   brighten up, brush up, do up, fix, fix up, freshen, furbish,
   furbish up, improve, perfect, polish, polish up, renovate, repair,
   retouch, revive, rub up, shine, spruce, spruce up, tone up, vamp,
   vamp up