$ sudo apt-get update
$ sudo apt-get install apache2-suexec 

apache2-suexec is installed in your system.

Make ensure the apache2-suexec package were installed using the commands given below,

$ sudo dpkg-query -l | grep apache2-suexec *

You will get with apache2-suexec package name, version, architecture and description in a table

apache2-suexec package basic information: