I was just wondering if i can downgrade the screen resolution of nexus 5 to 720p instead of 1080p can it improve the battery life ?
is it possible to achieve this ? If yes than how ?
No, it wouldn't make a lot of difference to the battery life, and it's just as likely to increase power consumption as to decrease it. The screen would still draw the same amount of power. All that you'd be saving is that the GPU wouldn't have to do as much work to shade pixels (i.e. determine the colour of each individual pixel) - but that would come at the cost of having to scale up each 720p frame to the 1080p physical resolution of the screen itself.
You'd only get a net benefit from that in certain kinds of game where pixel shading is very expensive. Such games often do this already - they render expensive parts at a lower resolution and upscale - to keep within the 16ms budget for rendering each frame. At all other times, the upscaling would be unnecessarily costing you power.