If you have forgotten the version of your Ubuntu OS then follow this tutorial which explains how to find the Ubuntu version.
Open the Terminal (keyboard shortcut: Ctrl+Alt+T)
In the terminal type following command:
lsb_release -a
Your version will be shown on the Description line.
hostonnet@hostonnet:~$ lsb_release -a No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 16.04 LTS Release: 16.04 Codename: xenial hostonnet@hostonnet:~$
So we easily find our Ubuntu Version is 16.04
You can also use
cat /etc/*issue
Command
On RHEL
cat /etc/*release
