Android upload key mismatch on an existing (acquired) Play app

I acquired an existing app on Google Play (package: com.getloomi.loomi) and rebuilt it with Median. The app already had Play App Signing enabled, so Google expects the AAB to be signed with the previously registered upload key. Google rejects my Median-built AAB with:

The app bundle was expected to be signed with the certificate with fingerprint:
SHA1: 55:DD...
but the certificate used to sign it has fingerprint:
SHA1: 90:FE:....

So Median signs with the key ending 90:FE..., while Google's registered upload key for this app ends 55:DD...

I think I can fix this by Downloading the keystore Median uses (the one matching 90:FE...), so I can request an upload key reset in the Play Console and register Median's key.

Where do I manage the Android signing keystore for an app ?