You can adjust it slightly (only as displayed in Settings though) by adjusting the name in the Microsoft account via browser. It won't adjust on the sign-in screen or other places that way.
The name used in whole everywhere is created by the email used so if Darth Vador used an email of ******@outlook.com then the Windows user would have the name and folder LUKE.
The only semi-official way (there's no true official way) to change this is to add the email desired to the Microsoft account and then add it as an alias in the account. Then you'd have to switch it in Windows. I tested this previously and it works. If you want the User folder name to change too then you have no choice but to create another Windows user using the new email as the account email or try the problematic hacky workaround for that.
Frankly, I wouldn't use the hacky workarounds like John linked unless you want to have a higher chance of problems. It would be better to skip it and just live with it how it is.