From my notes from my laptop which uses LUKS (I'm assuming your box does
too) these steps exist:

Code:
cryptsetup is the tool to do use.

#First, verify device is LUKS device (returns zero, or add -v to be verbose):
sudo /sbin/cryptsetup -v isLuks /dev/sda2

#Now show used slots, just so we know what is going on:
sudo /sbin/cryptsetup luksDump /dev/sda2

#Next change the LUKS key for that device AFTER entering the prompted-for
existing key for that slot
sudo /sbin/cryptsetup luksChangeKey /dev/sda2 -S 0  #where zero is the
target slot number, the first of eight total available
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Good luck.

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