Onlyfun You can temporarily install another app that opens APK files. (Random example: APK Explorer)
Then when you tap on an apk file android will ask you which app to use. Click 'Just once' instead of 'Always' and it'll ask you every time.
Then you can uninstall the extra app.
Edited to add: If your phone only has one app that can handle APK files then android won't prompt you to choose, since there isn't really a choice.