First, you need to download and install SP flash tool
Download and install MTK Android USB drivers
Then download Firmware file/Flash file/ROM for your phone
Your phone must have 50 % charging before this process,
switch off your mobile remove the battery (if removable) reinsert the battery and plug the USB cable in charging socket
Now open SP flash tool.exe
When SP flash tool open, go to download button in SP flash tool
In download tab, click on scatter load button
After locating the scatter tab, click on the download button
Now connect your smartphone with a USB cable and press volume up or volume down key for easy detection (remove and re-insert your phone battery if it is removable)
Now flashing is completed, a green ring will appear in SP flash tool
Close SP flash tool on your computer and unplug phone
Power on the phone and enjoy
After flashing process, starts, it might take up to 5-10 minutes. So, keep patience and wait.
See detailed Instruction MT flashing SP tool with images