Welcome at AppyBuilder.
You can find info here:
In principle. If you start a fresh app with appybuilder your app gets automaticly signed with a so called keystore. If you keep using appybuilder and update your app you don’t have to do anything else.
As an example i made an app with App Inventor and put it on Google Play. When i started using AppyBuilder for the same app i imported the keystore that i used on App Inventor in AppyBuilder. When i compiled the app i could update the apk in Google Play without any problem. I use App Inventor, AppyBuilder, Makeroid and Thunkable. They all use the same keystore.