You can change your application's plan by going to your application's editor, clicking on Settings, then on the App Plan tab, and clicking 'Change plan'.

Next, select either the monthly or annual billing frequency. Note that the default will be monthly, unless 'Annual billing' is toggled on. Then, select the plan of your choice and click the 'Upgrade' button underneath that plan.

If this is your first time subscribing or you don't have a valid payment method on your account, you'll be prompted to enter your payment information. Otherwise, you'll see your plan selection and pricing listed for review on the right hand side of the screen, and you'll just need to confirm the plan change.
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Upgrading or downgrading a plan part-way through the billing period will result in your account being credited for the unused time on the plan. Please note that when downgrading to Free, your app plan will stay active through the end of your billing cycle (and will continue to consume workload units), thus no credit will be issued.
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Please also note that the Annual subscription is charged in full for the whole year at the time of upgrade, and billing frequency cannot be changed back to Monthly once the Annual subscription is active.