shows update but just spins looking for updates in software update
unable to update. in software update spins at looking for updates, does not complete or error message
how do I update
MacBook Air 13″, macOS 10.13
Several workarounds have been proposed, such as restarting your Mac in "Safe Mode" and checking for updates while in that mode, and / or resetting NVRAM.
How to use safe mode on your Mac - Apple Support
Reset NVRAM on your Mac - Apple Support
Related: Software Update stuck at "Checking for updates"… - Apple Community