Punch set from PSI. Knock out the cap and clip first, then use a max size punch that first the tube, and knock out the collar. Carefully the insert of the collar is plastic try not to mangle it if you plan on re- assembling.
Use the punch set from Harbor freight though. It is so cheap but certainly great for the job. Only like $8 after discounts. I can disassemble almost any pen now.
HF punch set is fantastic, even better if there's an outlet within driving distance so you're not paying shipping. Otherwise sorry, I have nothing to add that wasn't already said