Hi Patricia,
Generally, Microsoft 365 for business subscription is associated with your work or school email account, do you have the admin account to log into Admin center?
I find the articles below, which describe how to change Microsoft 365 for business plans manually, I think they may be helpful. The first one applies to the old admin center, the second one applies to new admin center.
Change plans manually
Switch Microsoft 365 for business plans manually
As we are mainly responsible for general issues of Office desktop applications, but your question focus more on Microsoft Admin and subscriptions, it's better to contact Microsoft to have a confirmation.
For more related information, please refer to "Upgrade to a different plan".
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