Sony XPERIA L C2105 - Android 4.2.2 - firmware 15.3.A.1.17

It is following 5 guidelines showing HOW TO: 
1. install the newest Android 4.1.2 for your phone (for Sony XPERIA L C2105 it is the 15.0.A.2.17 firmware)
2. install the newest Android 4.2.2 for your phone (for Sony XPERIA L C2105 it is the 15.3.A.1.17 firmware)
3. backup your original firmware customization (it is important in case you must send your phone to service) 
4. unlock the bootloader (in case you want to install the custom kernels with recovery menu and more) 
5. relock bootloader on your phone

1. how to install the newest Android 4.1.2 for Sony XPERIA L C2105a) first of all, check your firmware version (if you already have the newest one, you do not need this tutorial)
b) download and install latest Flashtool from this article
c) download also latest Android 4.1.2 (mirror Google Drive) for Sony XPERIA L C2105
d) paste the ftf file downloaded in step c) to folder „Firmware“ included in the XPERIA Flashtool
e) open the XPERIA Flashtool, click on flash and choose downloaded firmware, than click ok. Important - if you do not want to delete any data of your phone, just uncheck flashing userdata.sin file. All data will remain in your phone, but sometimes it can cause additional troubles. So it is better (and I recommend that) to do the backup of your personal data, than proceed with clean instalation and then restore your personal data.
f) turn off your phone and when Flashtool shows pop up window, connect phone to PC with holding volume down button 
g) after few minutes all is done and your phone gets the new firmware

2. how to install the newest Android 4.2.2 for Sony XPERIA L C2105
a) first of all, check your firmware version (if you already have the newest one, you do not need this tutorial)
b) download and install latest Flashtool from this article
c) download also latest Android 4.2.2 (mirror Google Drive) for Sony XPERIA L C2105
d) paste the ftf file downloaded in step c) to folder „Firmware“ included in the XPERIA Flashtool
e) open the XPERIA Flashtool, click on flash and choose downloaded firmware, than click ok. Important - if you do not want to delete any data of your phone, just uncheck flashing userdata.sin file. All data will remain in your phone, but sometimes it can cause additional troubles. So it is better (and I recommend that) to do the backup of your personal data, than proceed with clean instalation and then restore your personal data.
f) turn off your phone and when Flashtool shows pop up window, connect phone to PC with holding volume down button 
g) after few minutes all is done and your phone gets the new firmware

3. how to backup your original firmware customization
a) if you do not want to lose warranty of your phone, first thing you have to do is to backup your original customization
b) download Sony PC Companion and connect your phonec) select update your phone in Sony PC Companion
d) application will prompt you to disconnect your phone before starting downloading firmware, just make it (remember - dont disconnect your phone when update just start...)
e) after finish downloading firmware cancel the update
F) after that, go to C:\ProgramData\Sony Mobile\Update Engine\db\some number\blob_fs, there you find 3 (maybe 4) files, which names start FILE
g) download and install latest Flashtool from this articleh) paste the three files, which names start FILE, from blob_fs folder to folder „Firmware“ included in the XPERIA Flashtooli) open the Flashtool, click on Advanced, choose decrypt files and put all three files from blob_fs folder to the right colummj) click ok and after approx. one minute a pop up Window will appear via Flashtool
k) mark all files except simlock.ta and transfer them to the right column, than fill in a box in the right upper corner (fill in what you want, it is just for you)
l) now click ok and Flashtool will create backup of your original firmware customization, that you can anytime flash to your phone

4. how to unlock your bootloadera) download and install latest Flashtool from this article
b) start Flashtool and click on BLU button on the top
c) turn off your device and when Flashtool shows pop up window, connect device to PC with holding volume down button
d) Flashtool will check your device and than you need to connect your device once more, now with holding volume up button
e) click on get unlock key button, it will switch you to Sony Mobile website, here select your phone and hit continue
f) fill your email adress and accept terms and conditions, than click submit
g) go to your mailbox and click on Click here to proceeed link in mail from Sony Mobile
h) copy IMEI from Flashtool to Sony Mobile site and once more accept term and contidion, than click submit
i) copy unlock key to Flashtool window and than click unlockj) your device is unlocked

5. how to relock your bootloader
a) download and install latest Flashtool from this article
b) start Flashtool and click on BLU button on the top
c) turn off your phone and when Flashtool shows pop up window, connect phone to PC with holding volume down button
d) just click on relock button and your bootloader is locked for now