So, first of all, find out which device is the encrypted system partition

you do this by running

cat /etc/crypttab 

it will be something along the lines of

nvme0n1p3_crypt or sda3_crypt

for our purposes we only need the drive name, not the _crypt so then from the results above we would only want

nvme0n1p3 or sda3

For the rest of the guide I will be using nvme0n1p3 as the example

Once you have the identifier you can do the following

sudo cryptsetup luksDump /dev/nvme0n1p3

This will show you how many keys are free, you can assign 8 (numbered 0-7)

If there are free slots you can assign a new password to unlock the machine

To do this you run the following command

sudo cryptsetup luksAddKey /dev/nvme0n1p3 

It will prompt for an existing password (as authentication) then it will ask for a new password, and then it will ask to confirm the new password, once complete, you will be able to unlock the system with the new password

If you wish to remove a password from the keystore, run

sudo cryptsetup luksRemoveKey /dev/nvme0n1p3 

enter the password you wish to remove, after a few seconds, job done

If you wish to change a password, run the following

sudo cryptsetup luksChangeKey /dev/nvme0n1p3 

then enter the password you wish to change, followed by the new one twice (initial entry and then verification)