Set from other PDF Viewer. Every PDF viewer provides an option to set it as default PDF reader. Therefore to un-associate PDF file types with Foxit Reader, simply open the viewer you desire and set the option there.
1. Right click on any PDF file you try to open, choosing open with
2. Select any PDF Reader you try to make as default reader
3. Select the checkbox Always use the selected program to open this kind of file, then Foxit Reader is no longer the default reader.