Tenant Office 365 admin user

Question

My customer has paid for an Office 365 contract until December with old supplier, but wants to have administrative control of the tenant, allowing him to perform various operations such as SharePoint and other tasks. However, the previous supplier refuses to grant him administrative access. I need to change the password for the administrative user ******@studiozarcone.it, or assign global administrator access to the user ******@studiozarcone.it . Can you help me ? thanks 

Regards 

Moved from: Microsoft 365 and Office | Subscription, account, billing | For business | Windows