If you have both an EndNote Online account and EndNote Desktop installed, you can switch between them when using the Cite While You Write plug-in in Word.
Why might you want to do this?
In Word, you will see EndNote on the ribbon at the top of the screen. Here it is defaulting to EndNote Desktop (the latest version is EndNote 20)
To switch to your EndNote Online account, select Preferences and then Application
In the Application tab, use the drop down menu to switch between EndNote (Desktop) and EndNote Online. Enter your email address and password for EndNote online and tick the box to remember your details. Select OK.
You will now see that the ribbon in Word has changed from 'EndNote 20' to 'EndNote' showing that it is now set to your EndNote Online account.