Simply changing your passphrase on a LUKS drive with only one passphrase is extremely easy. Open a terminal and run the following command, substituting the actual drive location for “sdX.” First, you’ll be prompted to enter your existing passphrase. Then, you can create a new one.

cryptsetup luksChangeKey /dev/sdX

Source: https://www.maketecheasier.com/change-luks-encryption-passphrase/