It's easy enough to untwist one end and re-twist it so the string is a little shorter, but I don't know that I would want to do that if I didn't already know how to make flemmish strings. So hang onto the string that is a little too long, and after you are comfortable making strings, you can shorten it if you want to. Otherwise, strings are not that expensive, probably $10-20, so you might just want to get a new one.
If they sent you a string that was too long, or stretched too much, with a new bow, they should be willing to replace it with one that is the correct length.