Switching from a monthly to yearly RememBear subscription
Head over to the the upgrade page while logged in and switch your plan to the Yearly option. You'll be automatically refunded for the remaining portion of your month and charged for a yearly subscription right away.
Switching from a yearly to monthly RememBear subscription
You'll need to cancel your current yearly subscription first, see here. Once your current year has expired you can re-upgrade to a monthly plan.
If you pay for RememBear via the iOS on Android app stores
Mobile plans are managed via their respective app store. If you have a subscription paid for via Apple, see here. If you have a subscription paid for via Google see here.