Apart from resetting your device via the Settings menu, there is another way to let your Lumia 900 perform its Master reset, you do it with the hardware key.
Hard reset may come in handy in case you would like your Lumia 900 to erase all your downloaded contents, including your update ringtones, pictures saved, apps, or contacts list stored in the Lumia 900 internal memory.
Do these followings to know how you can perform a master reset on Lumia 900 using hardware keys:
1. First, never forget to back up all your valuable data by copying any files you want to save from your Nokia Lumia 900 to your PC.
2. Next, power off your phone.
3. Now, press and hold the Volume Down, Power and Camera keys at the same time.
4. Once you feel the phone vibrates, release the Power key.
5. Do not release the other two buttons and keep them for 5 more seconds.
6. Now, release the Volume Down and Camera key.
7. You should now see Lumia 900 reset and restart automatically.
8. Once finished the restart, you will see your Lumia 900 is now returned to its factory default settings.
Note: Although you make a hard reset on Lumia 900, all data stored on the SIM card (if available) will not be removed.