How to create a server instance of just ComputerName instead of ComputerName\SQLEXPRESS
You cannot change the Instance Name after it has been installed.  But you can uninstall that SQL Server instance and reinstall as a DEFAULT instance.  Ideally you should make a backup of the server, then restore the backups to the new server.   

You can technically detach databases and attach again after the reinstall, but that includes the risk of losing your detached databases. See Aaron Bertrand's post: 

https://blogs.sentryone.com/aaronbertrand/bad-habits-file-backups/

When installing a default instance of SQL Server Express at the Instance Configuration page, you must choose the Named Instance radio button and type in the default instance name of MSSQLSERVER.  

All default instances use the MSSQLSERVER internally, but they just do not make it visible to you.