Please need help, badly needed!

Connect your mac to your router via cable rather than WiFi. Download the standalone Combo update from here,

https://support.apple.com/kb/DL2028?viewlocale=en_US&locale=en_US

Do not use the mac while it is downloading, and do not use other devices that share the internet connection.

Have you tried rebooting your computer and trying again?

Are you able to connect to the internet without any issues using a web browser?

If you are using WiFi, then try using an Ethernet connection instead as it is faster & more reliable and it bypasses a number of possible WiFi issues.

Try updating while in Safe Mode.

If you have any anti-virus or other third party security apps installed, then they may be interfering or blocking the update. Run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper.