In order to hard reset a Karbonn A21, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Karbonn A21 device will be brought back to the default configuration of a brand new device.
1. Power up the phone by press-holding the Power button.
2. Press and hold the Volume Up & Power buttons simultaneously.
3. Let go of the Volume Up button when the screen turns red.
4. A new menu will open. From it, select the wipe data with factory reset and yes options. Confirm them by holding down the Power button over each one.
5. Lastly, select reboot.
6. Yaaay, wheee, woooo, you've completed the hard reset process.