Log on with the new user account that you may have created. Make sure that this new user account has Administrator privilege.

Open Explorer > Organize > Folder and Search Options > View tab > check Show hidden files and folders box > OK to save settings.

Now navigate to your C:\Users\(Old Username) folder and copy all the files and folders in the C:\Users\(Old Username) folder to the C:\Users\(New Username) folder. To copy all these files and folders easily, use Ctrl+A, and then Ctrl+C. To paste them use Ctrl+V.