Change Add to cart button text in Drupal 7
The string overrides module is handy for these kind of things.

Provides a quick and easy way to replace any text on the site. 


  Features
  
  
  Easily replace anything that's passed through t()
  Locale support, allowing you to override strings in any language
  Ability to import/export *.po files, for easy migration from the Locale module
  Note that this is not a replacement to Locale as having thousands of overrides can cause more pain then benefit. Use this only if you
  need a few easy text changes.