Hello
If you go to Settings > Accounts > Sign in options, you should be able to find the change pin option to change it there.
If for any reason that option is grayed out or missing,
Can you go to your Windows Settings > Accounts > Access Work or School (note, it is the briefcase icon, not the person icon). Then see if the organization email address is still listed? If so, click on it and then disconnect it.
Restart the computer, and see if you can now change it.