Why won't my macbook air update?

Why won't my macbook air update? it's software is currently the OS X YOSEMITE (10.10.3) and I am trying to update it to 10.10.4. When I try to update it, it will tell me to restart my macbook and when I will click restart it would open App Store and tell me that there's Available update changes and I should click "show details". then when I do, nothing happens. 😐