The 'standard' way to get the Market/Play Store to update is:
- go to Settings/Applications/Manage Applications/All (tab)
- find Market/Google Play Store
- press 'Force Stop'
- press 'Uninstall Updates'
- exit back to home screen
- I usually reboot at this point, probably not necessary...
- run the Market/Play Store
It should ask you to accept the terms/conditions of the store. It won't update immediately, but it should start getting the latest update in the background. If you access the market in an hour or so, it should be updated (if the phone is able to access the internet, of course).