I updated my app to the Google Play Store last night. Before the update, my App version was 2.3. There was some problem in version 2.4 so after another 2-3 hour, I updated
Go to "Settings" within android device then on one of the tabs selections it should say "All" press that then look for the "Google play store".
Once you press that you can check what version play store your phone is currently running to make sure if it's running version 2.3 or 2.5.
I had a similar problem and it turned out that there is a "glitch" with the update button in that it desappears in landscape mode, and is only visible in portrait mode. When I searched to find out what this was about, I came across this page (we are on), so I thought I'd post my answer here for anyone who might also be stumped and might stumble here.
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
In my case, the update button shown after I restarted the phone or restarted the playstore
In order to show an update in Google Play, your app either should have been installed through Google Play or it should be a system application.
If you have installed your application through adb or installed from other sources, Google play will show "Open" instead of "Update".
There are the following use cases that you can try.
It will update button after some time, so wait for some time
Mostly it is a catch issue. So try to open/close the app several times. (Worked in my case)
At the current time(Aprin-May 2020) it is taking almost 4-5 days to publish app due to COVID-19. I checked in play console, the status is "Your app is published" but I see an actual update after 2 days.