How do i update my macbook if there aren't any updates mentioned? I'm still on version 10.12

Once Apple moves on from a major operating system release, any security or browser updates for that operating system, will not present themselves as an update until you have updated the operating system to its last update. In the case of Sierra, that would be 10.12.6. If you want the last update for Sierra, then you can download the combo updater, macOS Sierra 10.12.6 Combo Update. You would apply any Mac App Store updates that appear after the combo update has completed.

If your goal is to advance via upgrade to the latest operating system that is available for your Mac Book Air, you will have to compare the parenthesis content from the menu > About This Mac panel's product description line, with the hardware requirements found in how to get old versions of macOS. You would open that link only in Safari, so. you could successfully download the correct installer from the Mac App Store.