If you have an iOS "one instrument" subscription and wish to upgrade to "All Instruments" (Guitar + Piano + Ukulele + Bass), you are able to do so by going to "Settings" >> "Profile" >> "Edit Profile":
This will automatically open a new Window on your default browser that will allow you to edit your subscription:
The remaining value of your current subscription will be deducted from the full yearly amount and you will need to pay the difference. Enter your Email and Credit Card details and press "Upgrade Subscription". The upgrade will be effective immediately after purchase.
(!) Please note that you will have to cancel your ongoing iTunes subscription after the upgrade. Click here for more details.