Punching Machine OMERA OPRI 120 C

Hello, I just bought Punching Machine OMERA OPRI 120 C (picture attached), i am changing it to cnc. This might be a silly question but this is my 1st time working on a punching machine. I cant change the upper die, i loosen the clamp all the way, but the punching die moves up and down freely but it won't get off. do i need any special tool to change the dies? or how do i take off the dies? thank you
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