You can change the password only if the library state is open or changed. If the user library is in the installed state, open it with Object > Library > Open writable....
Before setting a read-only password, you have to set a write password.
Select the library in the navigator
Object > Library > Change write password...
The Change write password dialog box is displayed.
Object > Library > Change read-only password...
The Change read-only password dialog box is displayed.