As earlier answered suggested first its need to check there must be app is signed or downloaded from play store.
If it is download from play store even its not showing update button then follow below steps
Ensure you're connected to a Wi-Fi or mobile data network.
Clear cache for Google Play Services app (Go to Settings > Apps or Application Manager > ALL > Google Play Services > Clear cache > OK).
Clear cache data for the Google Play Store app.
Uninstall and reinstall the Play Store updates. (Go to Settings > Apps or Application Manager > Google Play Store app > Uninstall Updates > OK). As long as you have a strong Internet connection (Wi-Fi or mobile data), your Play Store app will update to the latest version within a few minutes.
Install system updates. To do this, try the steps listed below:
Open your device's main Settings app.
Select About or System updates (depending on your device, this may be different).
If an update is available, follow the on-screen instructions to install it on your device.
Now, please check if the update button is available.
Restart your device.
Also, you can check update available or not from Side Menu->My Apps and Game->Update Tab. here you can see all apps which have new updates