1. First of all, you need to check whether the Samsung USB drivers are installed properly in your computer to carry out the following process.
2. Next, you have to create a complete backup of all necessary data’s in your smartphone, so that your data will be safe and you can restore it later.
3. Then, check battery percentage level of your smartphone and make sure that it is 70% or above to proceed with the following process without any power issues.
4. At last, Enable USB Debugging in your smartphone by following this path: Settings -> About phone (tap on the build number to enable Developer options). Once done, go to Settings -> Developer options -> USB Debugging (tick on the box) and enable it. Also make sure that CWM recovery is installed in your computer.

After finishing all above prerequisites, you can proceed with the following tutorial.

Tutorial To Update Galaxy S2 GT I900G Android Phone:

1. To get started, you need to download Android 4.4 Kitkat Firmware and Google Apps Package on your computer.
2. After downloading is finished, you have to connect your device to computer by using Original USB Cable and transfer the downloaded zip files in your smartphone. Don’t transfer these files on your device sub folders.
3. Now, you need to disconnect your device from computer and boot it into the recovery mode. To do this, switch off your Galaxy S2 smartphone and then press and hold Volume Up button + Menu button simultaneously until it become active.
4. In recovery menu, you have to choose “install zip from sd card” and then select the Firmware package that you have already placed in your device (Step 2).
5. Once the firmware is updated successfully, you have to choose “go back” and then follow the same steps to install the Google apps zip package.
6. Once the flashing process is over, select “clear cache” option and once done select “reboot system now”.
7. The first time booting will take few minutes, so be patient.

Note: If your smartphone gets in to the boot loop mode, then you have to switch off your Galaxy S2 smartphone and then repeat the update process from the start. You can remove the battery from your device battery or long press the power button to switch off your smartphone.
That’s it. You have successfully installed Android 4.4 Kitkat operating system on your Galaxy S2 GT I9100G smartphone. If you have any issues while following this tutorial, do let me know via comments.