[Files]
; this will install (actually backup) the old files into a backup folder and after
; all of that is done, the CleanupDirectory function is called
Source: "{app}\res_mods\*"; DestDir: "{app}\_res_mods_{#DateTime}"; Flags: external skipifsourcedoesntexist createallsubdirs recursesubdirs; AfterInstall: CleanupDirectory('{app}\res_mods\*')

[Code]
procedure CleanupDirectory(const Path: string);
begin
  // note, that I have used no error handling, but you should; for how to DelTree
  // works, please refer to the help file
  DelTree(ExpandConstant(Path), False, True, True);
end;