Office 365 team extending its supports to add external users in many services like Office 365 groups, SharePoint groups, etc to provide collaboration experience with customers and clients. In Office 365, the guest users are represented by a mail user objects, so if you want to change email address for the existing guest user account, you can use the Exchange powershell cmdlet Set-MailUser.
Set-MailUser <mail_alias_of_guest_user> -WindowsEmailAddress <new email addresss>
Note:
If you don’t have enough admin privilege to use this cmdlet, you can just remove the existing guest account and add again with new email address.
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