If you purchased your Dipsea subscription through the iTunes App Store, you can change your subscription type (from monthly to annual or annual to monthly) on your iPhone by following the steps below:
Go to Settings on your iPhone
Tap Your Name at the top of the screen
Find the Dipsea subscription
Choose a different subscription option
If you're being charged for a subscription, but don't see it in your list of subscriptions, make sure that you're signed in with the Apple ID with which you originally subscribed to Dipsea. If this is the case, you may also have purchased your Dipsea subscription via the Dipsea website rather than the App Store.
If you purchased your Dipsea subscription via the Dipsea website, learn how to change your subscription type here.
To learn more about Dipsea's different subscription options, click here.