Sometimes in the Computer/Laptop, Windows is installed already with default Username already set. For users who are not satisfied with this and need to change this can do this using Group Policy Editor.
1. Open Run Dialog Box.
2. Open the Local Security Policy editor using “secpol.msc”.
3. Navigate to the tree Structure In the left pane, in Local Policies, expand it to Security Options.
4. In the right pane, there is a Policy, Accounts: Rename administrator account, Double click over it.
5. Change the name of Admin account to the one you wish to have now.
6. Close the Local Security Policy window, Log off to save the changes.