Can't Update Beta App on Google Play Store

I want to test new version app with my testers. I created close test channel and typed tester's email addresses. The beta app was published. App name was changed on Google Play Store to " (beta)". But we can't update. There is no update button on Google Play Store. There are two buttons, open and remove. How can we update the beta app to test on Google Play Store?

Thanks a lot.