Android App 21 Days Challenge full Download on Mobile Phone or Tablet PC with .apk file, with 100% Safety for Your Device! If you want to install 21 Days Challenge full on your phone or tablet you should do some instruction 😎: For First, You should go to the Settings menu, and then allow Install .apk Files from Unknown Resources. Don't be scared of that, our apk files are 100% safe and tested on millions of devices. Then you can load app from our store, and Install it on your Android OS Device. Sounds Really Easy, doesn't it? 😂