This is due to the level of access you have to the site.
You will need to confirm the permission level of the username you are using to log into your library site.
For example, if you are logged in with the 'Librarian' account or a user account that does not have administrator access you will only have the 'Change Password...' option under 'Security'.
You will also likely have less options from the 'Settings' menu.
When signed in with full administrator access you will have 4 options under 'security' on the menu bar.
If you are not logging in with an account with full administrator access, you will need to swap to the administrator username. If you do not know the password that goes along with the administrator account you can carry out a Reading Cloud or Junior Classic self-service password reset.