If you own a Sony Xperia Z3 or Sony Xperia Z2, then you should know that the Android 5.1.1 Lollipop OS can be installed on your smartphone by using some custom ROMs.
The Android 5.1.1 Lollipop can be installed on the Sony Xperia Z3 via a custom ROM created by MX, on the other hand, the Android 5.1.1 Lollipop for Sony Xperia Z2 has been created by NanoPop.
Keep in mind that both these ROMs are not official, which means that they could harm your device, at the same time, by installing a custom ROM to your Sony Xperia Z3 or Sony Xperia Z3, you will lose the warranties available for these devices.
In order to install a custom ROM on your mobile device, you will need to get a backup tool such as TeamWin Recovery Project (TWRP) or ClockWorkMod (CWM), at the same time, you will need to enable the USB Debugging Mode on your mobile device.
Installing Android 5.1.1 Lollipop on your Sony Xperia Z3 or Sony Xperia Z2
First of all, you will need to download the latest Android Lollipop update by MX zip file (for Z3 model) or by NanoPop zip file (for the Z2 model) to your computer, after that, you will also need to download the Google Apps zip file to your computer.
Once you have these two .zip files saved on your computer, connect the Sony Xperia Z3 or Xony Xperia Z3 to you computer via a USB cable. Once the mobile device is connected to your computer, copy the .zip files from your computer to your smartphone, after copying the files, disconnect the mobile device from the computer and turn it off.
Now, you will need to boot the device into recovery mode by pressing and holding the Power+Home+Volume Up buttons.
Once you are in recovery mode, you will need to wipe the data from the device, including the dalvik cache. Finally, after the device’s data has been cleared, get to the main recovery menu screen and select “install zip from SDcard” and choose the latest Android Lollipop update zip file that you’ve copied from your computer. Do the similar with the Google Apps zip file and after that, reboot the device by going to the main screen menu and tapping on the “reboot system now” button.