The Collaborative International Dictionary of English v.0.48:

Modify \Mod"i*fy\, v. t. [imp. & p. p. Modified; p. pr. & vb.
   n. Modifying.] [F. modifier, L. modificare, modificari;
   modus limit + -ficare (in comp.) to make. See Mode, and
   -fy.]
   1. To change somewhat the form or qualities of; to change a
      part of something while leaving most parts unchanged; to
      alter somewhat; as, to modify a contrivance adapted to
      some mechanical purpose; to modify the terms of a
      contract.
      [1913 Webster]

   2. To limit or reduce in extent or degree; to moderate; to
      qualify; to lower.
      [1913 Webster]

            Of his grace
            He modifies his first severe decree.  --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

modify
    v 1: make less severe or harsh or extreme; "please modify this
         letter to make it more polite"; "he modified his views on
         same-gender marriage"
    2: add a modifier to a constituent [syn: modify, qualify]
    3: cause to change; make different; cause a transformation; "The
       advent of the automobile may have altered the growth pattern
       of the city"; "The discussion has changed my thinking about
       the issue" [syn: change, alter, modify]