You can easily rename a Library in the web app, just as you would rename any document. You can rename local Libraries from the Mac app too, but only by removing it first and renaming it using Finder.
Follow these steps to rename a Library in the Mac app:
- Head to Sketch > Settings or press ⌘, and click on the Libraries tab.
- Right click the local Library you want to rename, and select Show in Finder
- Choose Remove Library
- Rename the Library using Finder
- Add the Library again via Sketch > Settings > Libraries (see Creating and Adding Libraries)
To rename a Library in the web app, click on 
on the Library’s thumbnail and select Rename from the menu. Enter a new name and save by clicking the Rename button.