This article covers how to check OS version using different methods. You can get more information of Ubuntu releases at official site of the Ubuntu Releases.
If you are using Ubuntu 16.04 then you can Upgrade using this guide Upgrade Ubuntu 16.04 to Ubuntu 18.04 LTS.
Ubuntu is a free, open-source Linux-based operating system, which has a long list of release versions. Finding out which Ubuntu version is running on your system can be important when troubleshooting issues or searching for installation guides.
To check the Ubuntu version, use the following command in terminal:
$ lsb_release -a
This will display you some details about your distribution including Ubuntu version:
Distributor ID: Ubuntu
Description: Ubuntu 20.04 LTS