I have downloaded the macOS Sierra from app store but unable to install it. It's not displayed in updates section...
Did you look in your applications folder? Usually it downloads there. If you open your Applications folder there's an app called Install macOS Sierra
After downloading macOS Sierra it doesn't appear in the Updates section of the App Store app. Instead it is downloaded to your disk and usually can be found in /Applications. If you have downloaded a public beta or a developer beta and moved it to a different folder previously, it may reside there instead.
Depending on your system language the name is Install macOS Sierra (English) or a name translated to your language accordingly.