Thank you for your question and reaching out. I can understand you are having query\issues related to change default name that displays in Emails.
If you are using Outlook then In Outlook, choose File > Account Settings > Account Settings -> Select the email account that you want to change, and then choose Change. -> You can change your name on the Account Settings screen. To change the name that displays when you send email, update the Your name field.
If you still can not change then you may need to any other IT or Tenant Admin of your corporate mailbox. ( https://learn.microsoft.com/en-us/microsoft-365/admin/add-users/change-a-user-name-and-email-address?view=o365-worldwide)
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