If you already have a version of Ubuntu on an iso file and want to have a new version of the same operating system; If your Internet connection is limited, you can use a utility to update the old iso file.
I will use zsync to download only what has changed in the operating system and then add it to my old version of Ubuntu.
To have zsync in your Ubuntu operating system, you must install it if you have not already done yet. To install zsync on Ubuntu, type the command below. When the Operating System asks you to enter your password, enter it to continue with the process.
sudo apt install zsync
Copy the iso file and replace the name of the new copy with the name of the version of Ubuntu that you want to update.
cp ubuntu-14.04.2-server-amd64 ubuntu-18.04.4-server-amd64.iso
Before you can update the iso file, you need to change the permission to allow writing to it.
chmod 755 ubuntu-18.04.4-server-amd64.iso
This permission allows the user and the owner to read, write and execute the file.
Now, to update the iso file, call the zsync utility and enter the absolute URL of the Ubuntu file which has the extension zsync.