First step to Install Firmware 

1. Download and install MTK  android USB drivers
2. Download And Install SP Flash Tool

How To Flash Wiko Rainbow

1. First, you need to download and install SP flash tool
2. Download and install MTK  android USB drivers
3. Then download Firmware file/Flash file/Rom for your phone
4. Your phone must have 50 % charging before this process
5. switch off your mobile remove the battery (if removable) reinsert the battery and plug the USB cable in charging socket
6. Now open SP flash tool.exe
7. When SP flash tool open, go to download button in SP flash tool
8. In download tab, click on scatter load button
9. After locating the scatter tab, click on the download button
10. Now connect your smart phone 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)
11. Now flashing is completed, a green ring will appear in SP flash tool
12. Close SP flash tool on your computer and unplug phone
13. Power on the phone and enjoy
14. After flashing process starts, it might take up to 5-10 minutes. So, keep patience and wait.
15. See detailed Instruction MT flashing SP tool with images