Is it face unlock you are asking about? If so, there is a handy guide here. It requires your phone to be rooted and have ClockworkMod Recovery installed. Here are the installation instructions from the link:
Before you proceed with setting up Face Unlock on your Galaxy S2, make
sure that your phone:
is rooted, has ClockworkMod Recovery installed, and is running Android
4.0.3 Ice Cream Sandwich firmware build XXKP8.
If your phone meets those requirements, you can follow these steps:
1.Download the Face Unlock update file to your computer and then copy the downloaded ZIP file to your SD card. Get the file here.
2.Turn off your mobile phone and reboot it into recovery mode. You can do this by pressing the Volume Up and OK buttons. Press both buttons
at the same time and then press the Power button.
3.If you are already in recovery mode, proceed by selecting “Wipe data/factory reset” and “Wipe Cache partition”.
4.Reboot your device by selecting “Reboot system now” from the recovery menu.
5.Reboot your device again into recovery mode.
6.From the menu, select “install zip from SD card”, then ” choose zip from SD card”, and then choose the Face Unlock ZIP file which is on
your phone. After you have selected the file, installation will start.
7.After installation, your device will again display the recovery mode screen. Select “Reboot system now” to reboot the phone. Take note that
this time, restarting your device will take much longer. On the other
hand, you can also opt to update this package from ClockworkMod