How to change a DARE app monthly subscription to an annual subscription with an iPhone?

Here are the instructions on how to switch your monthly subscription to the annual subscription:

Open the Settings app (gear icon).
Tap your name.
Tap Subscriptions.
Tap the subscription that you want to change the subscription type (in this case, the DARE app).
Under Options, what will show is the current subscription type that you have (there is a check icon beside it) and the other subscription types. Just tap on the one you want (in your case, the annual one).
Upon doing this, there will be a reminder on your screen from Apple saying, "Your new subscription will begin when your current one expires" then just proceed.