$ sudo apt-get update
$ sudo apt-get install libxext6-udeb 

libxext6-udeb is installed in your system.

Make ensure the libxext6-udeb package were installed using the commands given below,

$ sudo dpkg-query -l | grep libxext6-udeb *

You will get with libxext6-udeb package name, version, architecture and description in a table

libxext6-udeb package basic information: