Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Hi. I'm Brian and I'll try to help.
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Do you mean that you used a Yahoo address to create a Microsoft account? What steps did you take to try to change the address? Describe them in detail so we know exactly what you did.
If you say you can log into the computer, then you should be able to log into the account with a web browser. Please log into https://account.live.com/names/manage. What do you see there?