When Library Components (Symbols, Text Styles, Color Variables or Layer Styles) 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.
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.