Follow all the steps carefully
1. Download the stock rom v1 from here . Do not download stock rom v2 from anywhere, that rom contain many bugs.
2. Download sp flash tools form here. (tool needed for flashing rom on the device)
3. Download USB drivers from here
Now you need to install the usb preloader drivers you just downlaoded.
To install the drivers:-
1.Extract the USB drivers.rar file.
2.Switch off your phone
3.Open device manager(search for "Device Manager") or click on computer properties , then device manager..
4.Connect your phone in OFF state
5.There will be a device named "mt65xx preloader" which will be there for some seconds.
You need to be quick at this. As soon as it is displayed, right click and click update driver.
Browse manually the files from usb drivers you extracted.
NOTE: THIS METHOD WILL NOT WORK ON WINDOWS 8
Now, if you are using windows 8 do this to install the drivers:
1. Dwonlaod the usb driver.rar file.
2. Hover pointer to Top Right corner of your screen, choose Settings from sidebar
3.Click last option "Change PC Settings"
5.Scroll to end and find "Restart Now" under "Advanced Startup". PC Will be in troubleshoot mode now
6.Once screen is back, select option "Troubleshoot"
7.Select "Advanced Options"
8.Select "Startup Settings"
Now important step. There will be several options to select from. You need to select "Option 7". For this press "7" key from number pad.
After your PC starts, follow above steps.
10. Choose the win 7 file from usb drivers.rar file to install drivers on windows 8 .
4. Extract ROM and Flash Tool
5. Open Flash_tool.exe from SP_FLASH Tools.
6.Select "Scatter-loading" and select "MT6589_Android_scatter_emmc.txt" from ROM folder
7.Select "Download" option, it will search for phone now.(if does not detect your phone pull the battery out and put it in again.)
8.Switch OFF the phone.
9.Connect it with USB (in OFF state only) (proper divers must be installed as said above)
10.If all goes well you'll see data transfer. At the end there will be Green Circle in middle of screen which means ROM is flashed.(don't panic if there is no green circle just let it work for 10 minutes and unplug your phone from computer.)
11.Remove phone, Switch it ON.