How do I upgrade my macOS version from 10.13.2 to 10.13.6?

When I go to App Store to upgrade, there are zero updates available. I spent some time googling for answers, and found that it was recommended to download the macOS Combo update from the browser. I downloaded it, began setting it up, but when I got to "Destination Select" where you're supposed to select the disk you want to install the update software on, I wasn't able to select "Macintosh HD", which appeared as the only option. When I tried to choose "Macintosh HD", it told me: "macOS High Sierra 10.13.6 Update can't be installed on this disk. This volume does not meet the requirements for this update." This message was accompanied by a yellow triangle with a white exclamation mark inside. I can't figure out what this means and what I can do about it.

The reason I want to upgrade my software is because I want to install iMovie on my Macbook Pro (Mid 2010). iMovie requires a software of 10.13.6 or newer, but my macOS is only on version 10.13.2 (High Sierra). Any help is very appreciated.