An error occurred applying the software update

I have an iMac about 3 years old. I’ve been trying to update it since last night, but keep getting the same error. I’ve tried putting it into safe mode but it won’t go into it. And I’ve tried recovery which is a waste of time, and pressing the option button On start up but that’s no help as it just goes back to trying to install this crud update! HELP PLEASE!!!!