my macbook keeps asking me to update it.

Sometimes updates via the App Store can fail but still report that they have been installed in the 30 day list.

If you get repeats asking you to update then this is an indicator of this.

To check whether an update has been installed for definite, click on the Apple at the top left of your screen, then click About This Mac, then click on System Report> Software> Installations, click on Install Date twice so the list is now displayed with the most recent update listed first. If you cannot see the macOS update it has not installed.

If you want you can download it and install it manually from here, Download macOS High Sierra 10.13.6 Combo Update

once downloaded double-click to install.

it is often recommended when doing any large download either via the App Store or from Apple Support Download pages to download via a wired connection to your router as wifi can be slow and less reliable.

No the 10.13.6 combo is used to update your Mac to 10.13.6 from any previous version of 10.13.x

so no need to do incremental updates, just download the one I linked to.

Hello, does it say update what, or to what?

EtreCheck is a simple little app to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac.

http://www.etresoft.com/etrecheck

Are you running any anti-virus software?