From charlesreid1

Changing Encryption Passphrase on Encrypted USB Drive

http://www.cyberciti.biz/hardware/howto-linux-hard-disk-encryption-with-luks-cryptsetup-command/ - changing the encryption passphrase on an encrypted USB drive

Print some info:

$ fdisk -l
$ cryptsetup luksDump /dev/sdc1

Add the new encryption key:

$ cryptsetup luksAddKey /dev/sdc1

Enter your old passphrase, then your new passphrase twice.

Now delete your old passphrase:

$ cryptsetup luksRemoveKey /dev/sdc1

Enter the old passphrase to be deleted.