I have iPhone 5s (ios 9.2.1) and it has very serious problem about fast battery drainage so what I do??

There are two main reasons that cause your iPhone battery draining fast:

1. Low battery capacity. As time goes by, the capacity reduction of the battery is inevitable. No matter what method is adopted, it is useless, which is determined by the material of the battery.
2. High battery consumption. No matter how large the battery capacity is, the higher the battery consumption, the faster it drains.

Therefore, the first step need to be taken is to check the battery health. Navigate toSettings > Battery > Battery Health, where you can check your battery health. If the value is less than 80%, then it is suggested to contact Appleto change a new battery. If the value is higher than 80%, then you can follow some ways listed below to fix your fast draining battery.
Method 1:Reduce Screen Brightness.Drag down from the top-right corner of your iPhone screen or swipe up from the bottom of the screen to navigate to the Control Center panel to vertically drag the brightness icon to reduce brightness.
Method 2:Turn on Low Power Mode.This mode can help your iPhone to decrease background activities, which can be activated via the Control Center by hitting on the battery icon.
Method 3:Activate Dark Mode.The Dark Mode feature has been included in iPhone since the release of iOS 13, which can save a little battery life. Hit on the Settings app on your iPhone home screen and then find the option of "Display & Brightness" to click on. Find "Dark" and activate.
Method 4:Limit Apps Access Your Location.Fast draining battery can also be caused by apps accessing to your location for privacy aims. Go to Settings app, and hit on the option of Location Services and then choose a certain app to select when Location Services can be applied. If you find the app never needs to know your location, you can just choose the option of "Never".
Method 5:Limit Notifications.Many apps tend to offer you numerous notifications whenever you light up the iPhone screen. Go to Settings app and then choose the option of Notifications to click on. Thus, you can go through every app to decide if you allow an app to send you notifications by clicking on the toggle.
The methods above may save 10%-15% of your battery capacity. If you need more methods and detailed steps, then you can read on this post for further reference:12 Ways to Fix Fast Draining Battery

You can go Settings > General > Usage > Battery Usage to check the usage of the battery. Check what apps are eating up your battery and take some action.
This guide offers 15 methods to help you save iPhone battery, Top Tips to Save iPhone Battery Life, hope this can help you.

Usually fast drain can be caused by a virus, many things running like GPS, bluetooth