$ sudo apt-get remove libxext6 

Uninstall libxext6 including dependent package

If you would like to remove libxext6 and it's dependent packages which are no longer needed from Ubuntu,

$ sudo apt-get remove --auto-remove libxext6 

Use Purging libxext6

If you use with purge options to libxext6 package all the configuration and dependent packages will be removed.

$ sudo apt-get purge libxext6 

If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.

$ sudo apt-get purge --auto-remove libxext6 

See Also,