To upgrade to yearly subscription, it is as easy as going to your iTunes account and purchasing the yearly subscription.
Keep in mind that the yearly subscription will start AFTER your existing subscription (i.e. monthly) finishes.
It is important to keep in mind, that all purchases go through your iTunes account and all payments are handled via iTunes.
Some questions you may have?
The money I paid for the monthly will it be deducted from the yearly?
Unfortunately, no. The yearly will be a NEW charge from iTunes and will start when the monthly subscription finishes.