If you are the account owner, you can easily do it online in your account. If logged in here through your account, scroll to the top and click on the Account button in the red header.
This will take you to a summary screen. There will be a list of options in blue. I believe changing your phone number is the third one down.
You will be asked weather to change the number now or at the beginning fo the billing period. (I have done mine during a billing period without an issue. You will be given an option of prefixes. Make your choices and follow the prompts to call *228 Option 1.
I believe you lose your VM's but I am not sure.
I bought the phone and activated it but i dont know where to change it at
I updated my message above with instructions. But I missed the Prepaid part. That may not work. If not call prepaid CS and see what the options are.