Secondhand PC - Remove old Administator

My Grandson, a committed gamer, bought a new PC & sold me his old one.
The problem I have is the email address he used with, it which is 'locked' in to the PC as Administrator.
He hardly ever uses email & so he's totally forgotton the password.
The problem is that his name appears here & there & I'm unable to remove it/him.
It's not a major problem generally but there are a few things it affects.
For one, I'm unable to access Windows Store without his email password because it insists I use this Windows Hello thing.
I've attempted (but failed) to use my own credentials but it seems that his name/email is tied in with PC & without his credentials I'm unable to proceed.
I rarely use Windows Store but on occasion I need to to.
So, is it possible to remove his account from the PC?
Windows Home 21H1

Yes, I'm setup as Admin as well with an MS account.

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Yes, I'm setup as Admin as well with an MS account.
I go to MS Store & log in with my MS account but when I find an app & click on 'Get'
It opens the following:
Use Windows Hello with your account.
To easily & securely access apps with (grandson's email address) you need to set it up with Windows Hello Face, Fingerprint or PIN.
It then requires the password of Grandson'e email address.

Yeah, he tried that but to reset the password they require details he no longer has such as his DOB (which like many, he never enters correctly at set up) & the phone number of his long gone phone etc.
If that's the only way out of it then I guess I'll live with it.
Thanks

Thanks. I'll try that.

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Ah! That won't work because it requires the grandson's email password.
What's puzzling is that the two email accounts seem to be somehow joined.
For example, in Settings\Accounts\Email & Accounts\'Accounts used by other Apps' it shows the two email accounts: The grandson's & mine. If I click on his & select 'Manage' it opens MS switch accounts page & says:

grandson@gmail.com
Sign in
You're already signed in.

me@live.co.uk
Remain signed in with this account.
Sign out and sign in with a different account.

So that doesn't really get me anywhere.

Err... So I create a second MS account & upgrade it to Admin.
How will that act differently to my current Admin account.
Also, in Explorer C:\Users why does it it only show two folders: GrandsonName & Public? Why doesn't it show my Admin account that I'm currently signed in to?

Yeah, I figured that was the best way round it.
But then comparing that with putting up with messed up accounts...
I'll live with it as it is.
Thanks.