You can update an existing app.
The only issue can be for the Android version. When you publish an Android app you create/use a certificate named keystore certificate which has also an associated password.
If you have published this app by yourself you should have both the certificate file and its password (if you have used the APK Generator of your Siberian and you don’t have reinstalled your Siberian after having generated the APK of this app you still have them both).
If you don’t have these two items, certificate file and certificate password) you will not be able to update the Android version of your app but only to publish it as a new app.
For iOS there is no problem, you don’t need previous items to publish an update of an existing app.
