How can I remove the conduit top?

I need to either get the wires out of this conduit or push the wires futher up as I accidently cut them to short while removing my old water heater which could have been 27 years old.
I'm not worried about saving the thing if I have to drill it out.
It is a Regal 9672
The top as you can see from the picture is suppose do slide down but it won't budge.
I tried the simple things squeeing it in a vice, pushing it with a screw driver and prying it but no luck.
From what I can see the stop is simply suppose to slide down

replace it .

Either use a hammer and punch to slide it open, or pry the cover off.