how to update Mac OS10.13.6

Moving to a later version of macOS than 10.13.6 is called an upgrade, and is dependent on the age of your Mac and if it meets the hardware requirements of newer releases of macOS.

From the Apple menu, select About This Mac, and refer to the model and year in the parenthesis of that panel's product description. Compare that to the hardware requirements for later operating systems in the How to get old versions of macOS site. Use only Safari to access that site, and follow the link to the Mac App Store for the operating system where you meet the hardware requirements. Download that upgrade, which will appear in your /Applications folder as an installer.

Always perform a Time Machine backup prior to any operating system upgrade, and verify that your application, printer, and scanner vendors still support the upgrade operating system.