To replace the top of the cook top you will have to remove it from the installed position.
Disconnect the power and remove the cook top from the counter.
Place it face down on a protected surface.
Remove the screws securing the main top to the burner box bottom.
Remove the screws securing the control panel box and the conduit mounting plate to the burner box assembly.
Remove the main top.
Reverse the procedure to reassemble,verifying that the cook top frame is properly aligned.
Make sure the pieces of insulation,located int the front left and right corners,are put back in place. Failure to do so could create excessive cabinet temperatures.
Here is a link to your cook top model JED8430ADB. | 
Replacing glass cooktop on downdraft 
