My device is Asus X550LN Intel® Core™ i3-4010U CPU @ 1.70GHz × 4 GeForce 840M/PCIe/SSE2
Now i have installed nVidia Geforce 840m driver. should i need to install Intel graphics driver also ? If needed then how to install intel driver ?
Will you be using Intel Graphics for things that are of importance to you? If so, then go for it. To me, you'll be fine with your main graphics card. Intel graphics should be fine with the already included drivers in the Linux kernel. At least I haven't had any issues with mine for the time I have had on Ubuntu.