Step 1.Call Apple support or go the Apple store to check your battery. Firstly, the most important thing is to make sure that your battery is without problem, otherwise, things couldn't be easier.
Step 2.Find out what Apps consume battery life, get rid of it. Due to Apple's update, some apps may not get compatible with the latest iOS version that ate your battery life.
Step 3. Reset your iPhone 7. Settings>>General>>Reset>>Erase All Contents and Settings. Factory resetting won't reset your iOS version, you will be still on the same iOS version with before.
Step 4. Re-install System via iTunes. With the DFU mode activated, restore your iPhone 7 via iTunes. http://www.syncios.com/iphone/fix-iphone-7-7-plus-battery-draining-fast-and-overheating-issues.html
The creator of this guide has not included tools