Open Options (from the System Button) > Security to set your passwords.
Set the "Universal Password" (1) or the passwords for individual Classification Levels (2) by clicking "Set..." for the password you want to change.
Later on when a user wants to Authorize, Organize will verify him for the highest possible clearance. This means Organize will first try to check your entered password against the Universal Password. If the password fits, you get Universal Access to all data. If not it will check the next lower clearance (Top Secret) to see if the password fits. This will be repeated through all security levels. If the password doesn't fit on any level, the clearance will be revoked to level 0 (Unclassified only).
Enter the "Current Password" (1) to authorize the password change, if you're already set a password before. If you set the password for the first time, the current password is empty (not the word "empty", but just don't enter anything in the current password field).
There are no enforced rules for the password. Just enter any password you like as "New Password" (2) and again at "Confirm Password" to make sure there was not typo.
Now click "Set new Password" (3) to save the newly created password.
As always: You need your password to change it later on and to authorize. So keep it save!