How you update your subscription depends on which platform you're using and where you subscribed.
If you subscribed to YNAB on a computer, directly through YNAB, changes to your subscription will need to be made in the web app. You can change your email address and password on your mobile device.
Mac App Store
If you subscribed through the Mac App Store, you'll need to manage your subscription through Apple.
You can access your subscription information here via the App Store or iTunes.
For more information, take a look at this subscription management information from Apple.
Android Play Store
For those of you using the mobile app on Android and subscribed on the web or through the Google Play store, you can make any changes (except completely deleting your account) by going into Settings and tapping Account Settings. You can even purchase YNAB gift subscriptions!
Finally, if you want to use your shared YNAB subscription on multiple devices, simply login with the same credentials and you'll be set!