Just to be sure, you do know that Sprint is the only carrier that allows you to have the same mobile and Google Voice number, correct? If you port your mobile number from another carrier to GV it voids your contract and you would have to pay an ETF.
With that being said, if you want to change your number go to voice settings (click on icon on the top right) on the GV website. Under the phones tab it shows your current number and a Change/Port link
Edit: Posted after OPs last post