As your installation is really outdate, the steps involved to upgrade to the latest version are a bunch of work. Please follow the instructions:
from 2.x to 3.0:
from 3.0 to 4.0:
Additionally, you will face some issues with App Center after it has completed, as some applications are no longer supported and App Center will complain about this. In my opinion, it would be easier and cleaner to backup all necessary data to a external drive and reinstall Endless OS from scratch using the provided images.