First, sign out of Copilot. If you use the Copilot app on Windows, open Settings > Accounts > Email & accounts, select the old personal account, and choose Remove. For an old work or school account, check Settings > Accounts > Access work or school instead. Microsoft notes that removing an account from Windows may also affect other apps using that account.
If you use Copilot in a browser, sign out and clear the saved cookies or sign-in data for Copilot and Microsoft login pages, then restart the browser. This removes the saved account suggestion, but it does not delete the Microsoft account itself.
Also, avoid closing the account unless you want to permanently delete the account and its associated Microsoft data.
I hope this helps, please feel free to let me know if you need any further help.