Your EndNote library is automatically saved so you can just close the library or the application. If you edit a reference you will be prompted to save it before closing.
It is good practice to back up your EndNote library, particularly if you save your EndNote library on a network drive, such as your UCL N drive, as EndNote files on network drives often become corrupted. You are therefore advised to back up your library to a USB flash drive or external hard drive. Do not save your library in cloud storage such as Dropbox, OneDrive, Box, SugarSync, etc. as this can easily cause them to become corrupted.
Synchronising your library with EndNote Online also provides a backup, but it is still advisable to make regular back-ups of your library in case your online storage is insufficient to store associated full text files or EndNote Sync fails to work successfully.
An EndNote library is made up of a library file (.enl) and a data folder (.dat). Your library will not work unless both are saved together in the same place with the same name. To ensure you back up correctly it is therefore advisable to use the save options available within EndNote.