If you have used previous versions of EndNote (20, X9, X8) and have already set up sync with your EndNote online account you shouldn't encounter any issues with syncing your library with EndNote 21. Use your existing EndNote online account details to register for sync on EndNote 21. Once you have downloaded EndNote 21, open your previously synced library and follow these instructions from Clarivate.
In EndNote 21, from the main Library window, click Sync configuration and the Sync Now button to open the EndNote online dialog box. Click the sign-up button and begin the registration process following the online instructions.