You are searching for two different features of simulink:
Library
Mathworks-Documentation: Working with linked blocks
To edit a block "somewhere" you have to disable the link, make your changes and resolve the link again. After updating you model (ctrl+d) or updating the links (ctrl+k) the updated lib content is in every instance of the lib.
Masking of Blocks
Mathworks-Documentation: Simulink masking
Masking has nothing to do with libs. But you can mix up both features.