Upgrading your plan can be done by simply tapping the new plan you wish to purchase.
To downgrade or cross-grade (same plan moving between monthly <> annual)
- Tap the Menu icon in the top left corner
- Tap Plans
- Under the Current tab, tap on Manage your App Store Subscription here
If you are downgrading plans, your current plan will remain in place until your renewal date, then will be renewed with your new plan and price.