If you use Windows XP or below you can go to Start > Control Panel > Add or Remove Programs. In the Currently installed programs box, click on the magicJack program and then click Remove.
If you have Windows Vista or above you need to go to Start > Control Panel > Programs > Programs and Features. Select the magicJack application and click on Uninstall.
After that, run CCleaner or Glary Utilities to make sure that there are no leftover rogue registry keys and reinstall the software. In this way you will reset the application and remove the entered user name.