No option on App Store to update High Sierra to Mojave
Based on your post, your MacBook should be compatible with the Mojave update.
- Once the App Store app finishes downloading and installing you should see a installer in the Launchpad (it should automatically open, otherwise try double clicking on it to manually start the installer).
- If this doesn't work out you might want to look at making a bootable USB installer, and proceed with the installation after booting from the USB stick. See here (Apple Support Documentation)