
Hi @Leung Frederick ,
I suggest you refer to this support article about "forgot the username or password for the account I use with Office":
https://support.microsoft.com/en-us/office/i-forgot-the-username-or-password-for-the-account-i-use-with-office-eba0b4a2-c0ae-472c-99f6-bc63ee2425a8
You can also read this article to get more information about managing the Microsoft 365 subscription:
https://support.microsoft.com/en-us/office/manage-your-microsoft-365-subscription-959ac957-8d37-4ae4-b1b6-d6e4874e013f
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