If you already had a Microsoft 365 subscription and did not purchase it from the iOS app, you can manage it at: https://account.microsoft.com/services
If you purchased a Microsoft 365 Personal subscription from the iOS app, go to Settings > [your name] > Subscriptions.
Microsoft 365 subscriptions purchased from the app will be charged to your iTunes account and will automatically renew within 24 hours prior to the end of the current subscription period, unless auto-renewal is disabled beforehand.
To manage your subscriptions or to disable auto-renewal, after purchase, go to your iTunes account settings. A subscription cannot be cancelled during the active subscription period. Any unused portion of a free trial period if offered will be forfeited when the user purchases a subscription to that publication, where applicable.