When Library Components (Symbols, Text Styles, Color Variables, Layer Styles or Artboard Templates) you use in your document get updated, you’ll see a Library Updates Available notification in the notifications icon 
of the Mac app’s window. Clicking on this will bring up a Component updates dialog.
Select a Component to see what’s changed and check the checkbox if you want to update it in your document. If you don’t want to update a specific Component, uncheck it. When you’re done, click Update Components.
When you change a component in a Library and save a new update, you’ll need to download the most recent update of that Library too. It works just as with any other Library update: when a notification appears in click on Library Update Available to open the Settings window and download the most recent update.
To stop a Symbol from updating double-click any instance of it in your document and click Unlink from Library. This will create a local Symbol on your Symbols page instead.
Tip: For information how to unlink Text and Layer Styles, take a look at How to Edit Library Styles.