EndNote Online is a web-based, version of EndNote. Entries created in EndNote Desktop can be easily moved to EndNote Online and vice versa. See EndNote Online Quick Guide for guidance. It is easier to use the desktop version as your working EndNote Library.
To sync you need to create an EndNote Online account.
Click Sync Configuration and fill in your details to create the account.
You can synchronise one EndNote desktop Library with one EndNote Online Library. After you run the sync, all the references and attachments will be synchronised.
After the sync, you will see exactly the same number of references, the same groups and attachments in your EndNote Online Library.