EndNote X5 was not designed or tested for Windows 10 but can be installed and used with Windows 10.
You can find more information about this on our website:
Since you own a previous version of EndNote, you would be eligible for an upgrade version of EndNote X8. You can place your order through our purchase page (http://www.endnote.com/enpurchase.asp) or you may also contact our sales team for more information on upgrades and current offers. You can find their contact information here (http://endnote.com/info/sales-information-request).