Are you trying to change the email address of a synced account?
If this email address is synced from on-premises to Azure AD then you cannot change the email address attribute value directly on the Azure AD portal. It has to be changed and synced again from on-premises for a particular object.
If you are changing an email address of an object which was created on cloud, then you can change the contact information via Microsoft 365 admin center.
Reference: Change a user name and email address
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