You can change your Office 365 subscription and auto-renewal options in the iPad or iPhone App Store.
On your iPad or iPhone Home screen, tap App Store.
At the bottom of the screen, tap Featured.
Scroll to the bottom and tap the Apple ID button (or the Sign In button if you're not signed in).
Tap View Apple ID and enter your Apple ID password.
Under SUBSCRIPTIONS, tap Manage.
Turn off auto-renewal
Your subscription renews automatically at the end of each subscription period. To stop auto-renewal, turn the option off.
Cancel a subscription
To end your subscription, turn off auto-renewal. Your subscription will continue until the end of the subscription period.
Switch between yearly and monthly subscriptions
You can switch between the monthly and 1-year versions of the same Office 365 plan. For example, if you have a 1-year subscription to Office 365 Personal, you can switch to a monthly subscription to Office 365 Home. Note that your new subscription will begin when the current one ends. For example, if your 1-year subscription ends December 31st and you switch to a monthly subscription, the monthly subscription will begin December 31st.
Change Office 365 plans
To switch plans between Office 365 Home and Office 365 Personal, do the following:
Turn off auto-renewal and wait from your current subscription to end.
Open an Office app, open a file, and tap Go Premium when you see it.
Buy the Office 365 plan you want.