You need two cloth towels, and two adjustable wrenches. One for each end of the pen. Use the towels to protect finish. Using the wrenches to gently turn the click end using an upward motion at the same time. Turn the click end in the direction that causes the nib end to tighten (should be clockwise looking down on the click end of the pen), otherwise you'll just be unscrewing the nib. You could use a small nylon washer between the nib and coupler to help make it easier to get nib off afterwards, as you will be tighteng the nib when removing the click mechanism.
This is how a disassembled one that the tube was originally too short to work correctly.