When you export as NW.JS, you get a dialog for NW.JS options.
On top of it, you have a drop down list and a link "Manage versions" under it.
Click the link it opens a list of all available versions.
Right-click on the version you want (likely the first available in the list with the most recent date) and click "Download".
You should have a tab at the bottom left indicating it is downloading.
Make sure to uninstall previous versions already installed in order to make enough disk space for it. (Right click an installed version and "Delete" it).