You can clear defaults by going to Manage Apps, selecting the app, and then hitting the clear defaults button. (blog.the-ebook-reader.com)
Should you wish or need to, you can always restore your system to the Windows default settings using the Restore Defaults button and applying them. (thewindowsclub.com)
Then from the list of search engines, click the Make default button next to DuckDuckGo. (pcworld.com)