Hi @Helen
If you are an IT Administrator you will need to do this through Azure Active Directory. If you do not have access to this, you will need to contact your IT Department.
How to change the display name of a guest in Microsoft Teams
https://ucstatus.com/2017/09/19/how-to-change-the-display-name-of-a-guest-in-microsoft-teams/
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If this is helpful please accept answer.