Almost all citation management tools are able to import citation libraries from other platforms without much trouble (EndNote -> Zotero; EndNote -> Mendeley; Zotero -> Mendeley; Mendeley -> Zotero). The best way to do this is to EXPORT your current citation library as a *.xml or *.ris file.
Exporting your EndNote library as an *.RIS file:
1) Select the citations or library you want to export. Next, click on File then select Export:
2) You need to select Output Style. Choose where on your PC you want to export your citation library. Then click on Output style:
3) To ensure an *.RIS output, choose Select Another Style and search for and select RefMan (RIS) Export -> Choose:
4) Now that you have selected .RIS file type, you must now ensure that you Save as type Text File (*.txt). Next, add .RIS after the File name and then Save:
Your EndNote citations are now saved under your file name as a *.RIS file. You can now import into Zotero or Mendeley.
Exporting EndNote Library as XML file
You should also save your EndNote citation library as a *.XML file. Both Zotero and Mendeley accept XML format as well as *.RIS. To do this, follow step 1 above, but in Step 2, click Save as type *.xml: