How to Prevent auto firmware update

Beciasue the last update caused me so many issues I had to revert to an earlier version - So I would prefer to be in control of this. I wont wake up to what appears to be a shoddily tested piece of code being automatically updated on my router which causes it to drop and run so slowly -I thought I was on a dial up line. 

Can you tell me how to revent the firmware from being upgraded?

Thanks

Frank.