unable to install update

Try downloading the Combo update & applying it in Safe Mode..

https://support.apple.com/kb/DL2048?viewlocale=en_US&locale=en_US

Safe Boot, (holding Shift key down at startup).

When updating from Apples servers it is often best to have your mac connected to your router

via cable. If you can only use WiFi then move your mac as close to your router as possible.

Don't use the mac while it is downloading or any other devices that share the same internet connection.

Which exact update are you trying?