A bit of clarification:
- We’re talking about disc encryption (LUKS), not pin.
- You have already been able to unlock it before (so you don’t mean you need the first time passphrase).
If so, I suggest you use GNOME DIsks app, which is a gui app. If you need to install it, in terminal do sudo apt-get install gnome-disks and the “Disks” icon should appear. From there: select your 31GB Disk → select “Partition 2 31GB LUKS” from the Volumes table → from the menu button on the left (next to the lock), select “Change Passphrase…”.
Remember to write it down (to a password manager or somewhere safe)! Just because a change can be forgotten amids festivities and such - and there is no way to recover it.
Yes, Mint mobile works. See, for instance: Activating a Mint Mobile SIM for Librem 5