In order to hard reset a Siemens AF51, you should perform the following steps.

Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Siemens AF51 device will be brought back to the default configuration of a brand new device.

1. Heyoo. Start by turning on the phone with the Power button.

 

2. Type in your phone's secret code *#9999#.

3. Lay back and wait for the phone to reset and restart. Done!