Q: after install new update for iphone battery life goes down,why??/

Hello mdchaudhary,

Welcome to Apple Support Communities.

It sounds like the battery life has decreased on your iPhone 5s since installing the latest version of iOS. The article linked below provides a lot of great information that can help you investigate battery usage on your iOS device.

About the battery usage on your iPhone, iPad, and iPod touch - Apple Support

Also, this article provides a lot of great information and troubleshooting tips that can help increase the battery life on iOS devices.

Batteries - Maximizing Performance - Apple

Cheers.