In Aptana, Emmets snippets.json is within it's signed jar file, so editing it is kinda out of the question. What you CAN do, is extract it (or get it directly from Git @ https://github.com/emmetio/emmet/blob/master/lib/snippets.json), see which abbreviations/snippets you'd like to change, then use Emmets GUI interface in Aptana to add your own using the same names as the defaults you wish to override.
This allows you to customise how each snippet/abbreviations acts without having to bugger around the file system.