If you are currently using EndNote 20 and would like to upgrade to EndNote 21, there are a few steps you need to follow:
Save your library and save any customised files, e.g. output styles.
Uninstall the previous version of EndNote 20.
Install the new version (EndNote 21).
Your library will be opened with the new version of EndNote. You can now add the customised styles into your EndNote program. Alternately you can save your customised files into My Documents - EndNote - Styles/Filters/Connection Files (individual folders). Either way you will be able to use these files from within EndNote.