First, find out which is the encrypted LVM partition
cat /etc/crypttab
To add a new password, use luksAddKey:
sudo cryptsetup luksAddKey /dev/sda3
To remove an existing password, use luksRemoveKey:
sudo cryptsetup luksRemoveKey /dev/sda3
View currently used slots of the encrypted partition:
sudo cryptsetup luksDump /dev/sda3