The simple method to factory reset any Android device is as follows-
1. Tap on Settings>General Settings>Back up and Reset>Factory Data Reset
2. Choose whether you want to delete apps or downloaded content.
3. Confirm and follow on screen instructions.
If, due to any reason, you cannot access Settings, the Redmi 1S has Mi Recovery which can be used to reset your phone to factory settings.
Here is how you can access Mi Recovery-
1. Switch your phone off.
2. Press Power and Volume Up buttons simultaneously.
3. Release the button when you see Mi logo. Keep the Volume button pressed.
4. You will enter Recovery mode.
5. Choose the language using Volume +/- keys.
6. Once the language is chosen, you will get the options to Wipe all Data. Select it via Volume +/- keys and confirm with Power key.