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[Quarnero]

@dav69

how to change Lock Screen password?

@dav69, please read carefully, user related one (not partition related one):

@amarok

Use the passwd command in terminal.

Please use numbers (for time being) only:
$ man passwd
$ q −− type q to exit

EDIT:

@dos

LUKS passphrase can be changed in GUI via the GNOME Disks app (which is installed by default).