@Sara Wong - Welcome to Microsoft Q&A and thanks for reaching out to us.
To remove an admin from your Microsoft Azure Founders Hub account, you need to have the Global Administrator, Privileged Authentication Administrator, or User Administrator role assignment. If you have one of these roles, you can follow these steps to remove the admin:
Login to the portal and go to Azure AD and search for the user you want to remove as an admin from your tenant, select the role in role pane and remove from the role.
Hope this helps. and please feel free to reach out if you have any further questions.
If the above response was helpful, please feel free to "Accept as Answer" and click "Yes" so it can be beneficial to the community.