There are two basic ways to do it. One is modifying existing prompt sets, which can be problematic because they can get overwritten by a later update, or by copying the prompt set and creating your own.
See basic instructions here:
http://www.3cx.com/blog/docs/localize-promptsets/
If you were basically happy with the UK English prompt set, use it as a starting point. The particular prompt you're looking for is probably "RECYAMSG" - which in the UK English set says "Record your message and press hash or press star to contact the operator," but you'll have to go through the various prompts and make sure you find the one you want. You can find them all under Settings -> System Prompts
Enjoy!