When you update components in a Library, documents that use them will get an update notification. You can review those changes and confirm them to make sure you’re always using the most up-to-date components in your work. Open Sketch’s Preferences by choosing Sketch > Preferences (or pressing ⌘ + , and click on the Libraries tab. From there, you can either drag a Library Sketch document into the window, or use the Add Library… button to browse for it.