I will describe an example of the firmware LG X135 L60 through the program SP Flash Tool:
1) Install the driver, they can be found on the forum 4PDA
2) Download the firmware and the program SP Flash Tool, for example, the Official firmware Android 4.4.2 + SP Flash Tool
or they can also be found on the forum at the link above.
3) Unpack the downloaded archive and run the program SP Flash Tool, in the line “Download Agent”, specify the path to the file “DA_SWSEC.bin” (it is located in the folder with the SP Flash Tool). In the “Scatter-loading” line, specify the path to the file “MT6572_Android_scatter” (be in the folder with the firmware), wait a few seconds while the program checks the firmware files.
4) Remove the battery from the phone for a few seconds and put it back, click the “Download” button in the program and connect the switched off phone to the computer.
The program will start the firmware process, the status will be displayed below.
5) When the process is completed, a window with the text “OK” is displayed, after which you can turn off the phone.