Unable to install Mac OS Catalina Update 10.15.3 on my MacBook Air

I have been trying to update my MacBook Air with 10.15.3 update. When I select "Update now", it says "Restart required".....

And when I restart, it simply pauses for a second to install the update, then skips the installation and restarts.

How do I update my Mac ?

Thanks for your help in advance.