High Sierra update 10.13.6 won't install

My High Sierra is at version 10.13.3, and I got the 10.13.6 update waiting for me to download and install. I did download and install it, restarted; but I still get the 10.13.6 update waiting for my installation. Having repeating the update several times with no avail. I do not have any anti-virus software installed.

Any solution? Thanks

Macbook pro A1502 (13 inch, 2014)

This is a copied post from this link :

https://discussions.apple.com/thread/8472996

The guy solved it but i didnt quite understand is solution, can anyone break it down into even simpler to understand steps

this was his solution :

I successfully updated it by:

1. Create a bootable usb flash drive (8GB) containing 10.13.5 mac os which I downloaded a few weeks back (you may choose to download the latest version .6, but I chose not due to long downloading time)

2. Shut down and reboot by pressing Option key, choose your flash drive, then install it without erasing your main ssd drive.

After finishing the installation, reboot, all my apps and settings are still intact. I then opened App Store to further update it from .5 to .6 without problem (no flash drive needed).

Thanks for all your assistance.