How do I change my username in Win 10 with Microsoft account?

I have the following problem: when I start my laptop and log in, my user name shows my full name (first and last name). Can I somehow rename myself so that only my first name is there?

Change username: Windows 10 with local user account

If the control panel is not displayed, you can find it easily via the search bar in "Settings".
In the User Accounts area, click on Change account type…
Select the (local) account you want to change…
Click on "Change Account Name".

https://www.ionos.de/digitalguide/server/konfiguration/windows-10-benutzername-aendern/

5 seconds google.

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Windows 10: change username - how it works!

Press the key combination Windows key + R.
In the Run dialog, type control and press Enter.
Select the Change account type item in the User account chapter.
Now click on Change Account Name.
Enter the new name and click Change Name.

https://www.heise.de/...l#anchor_3

I only 3 seconds

I took my time because I had another sip of coffee.

Oh well. That explains everything. I just finished my coffee.

That means you already had a caffeine boost that inspired and stimulated you?

Yes and for better bowel movements

I just texted my coffee, I also like to have better bowel movements today!