Here is the article if you mean as below:

  • How can I switch from a subscription plan to a different plsn?

Changing the subscription plan from monthly to yearly or vice versa is way simple. Apple enables the subscriber to do this change from AppStore directly, please follow the same here.

Apple handles the effect of such changes. When you change the plan, the 'change' will be effective as per your new preferred frequency after completing the existing cycle. That is if your existing plan is monthly and 17 days are still remaining for the subscription to renew and you switch to the yearly plan, the yearly plan will be in effect when the current subscription month completes - after 17 days.

In other words, the change in plan will not happen immediately, if there are still a few days remaining for the existing subscription cycle to complete. In such a situation, after those remaining days are passed, the new subscription cycle will be in effect.