To change the passphrase for disk encryption, first click on "Activities" in the upper left corner and enter "disk" in the search bar. Then select the "Disks" program that appears in the middle of the screen.
Now select the field that says "Luks". Afterwards it should have an orange background.
Now left click on the gears and select "Change Passphrase" from the context menu.
Now enter "PleaseChangeMe" as the previous passphrase and select a secure new one.
Then you have to enter the password of your account.