Hold them down together for about 5 to 10 seconds. Do not release them until you see the appear on the screen. Once the logo appears, you can release the Power button but keep holding the Volume Up button until the Recovery Mode screen appears.
When you see the Android robot with "No Command," press and hold the Power Button , then quickly tap the Volume Up Button once. Release both. How to Hard Reset TECNO Spark 10C
Tap on Reset Phone (or "Backup & Reset"). Hold them down together for about 5 to 10 seconds