Hi,
After update to window 8.1, you tap on HERE Map. It will ask for install/update. You have to sign in your Microsoft account or create one. Then HERE Map will be updated, not only it but all the apps will also be updated automatically one by one for you (because apps must be up to date with new W 8.1). It will take about an hour or less depending on how many apps you have. Therefore, you should charge your phone while doing this. After all are done, open HERE Drive, tap on a "+" to complete it require and you will have gps again.
Happy Holiday!