If you have a partial download that's been sitting around for awhile, I'd delete what portion that it's downloaded which will sit in your Applications folder. It should be called something like, "Install OS X Yosemite." Or you can delete it from LaunchPad.
If you want, at this point, you can try downloading a fresh copy from the App Store.
On that model of MacBook Pro, you still have an Ethernet port. You can use a direct ethernet connection to your modem/router and see if that makes a difference since it's a 5GB+ download. It'll also probably go faster.
Safe Boot is a good option, too, to make sure there aren't any third-party applications getting in the way of the download.
If that still doesn't work and you have an Apple Store nearby, try going in and bumming off their Wi-Fi to totally rule out a network issue. Make a Genius Bar appointment, show up early and try the download, and if it still doesn't work, you'll be just in time to have a Genius take a look.