Manually uninstall Play Store (tap ‘Settings’ -> ‘Apps’ -> ‘Downloaded’ -> ‘Google Play Store’ -> ‘Uninstall updates’ -> ‘OK’). This will come back to previous Android Market. Then, update to Play Store again.
Once you have uninstalled updates, tap the Market icon either on the desktop or apps and you will be prompted to accept terms and conditions for Google Play Store. The update will then take place automatically.