Endless OS will ordinarily update automatically when internet connectivity is available. This page details how to manually check if an update is available, and install any such available update.
Open the App Center from the desktop
Click the Updates tab on the top bar
If an OS update is available via the internet, it will be displayed similar to the example shown below.
Click Download to start the download and installation process.
Open the Settings app
Click About on the left bar
Information about any available OS update will be shown.