With a Chrome Extension you can download APK files from Google Play Store to your PC and install or backup on your wish. The extension adds "Download APK" button next to Install button on PlayStore.
Why not just change the ROM (generally OS) of Low Budget Karbonn A21, A9+ Phones
Before you give your handset to repair shop or buy new handset, please try the following procedure to repair your device.
Below is the list of some of my favorites and useful android apps. The list is specially for memorable purpose and I cannot make guaranty about availability on play store all time.