In practice, changing the signing keystore for an existing app ID is usually not supported once builds have already been uploaded. The store associates the app with the original signing key. So maaaybe that's why you are not seeing the option anymore.
So I think that your options are:
- Continue using the original keystore
If you still have access to it, this is the easiest path. - Create a completely new app ID
This is what many developers end up doing if the original keystore is lost or compromised. - Contact Meta developer support
There may be an internal process for fully wiping uploaded binaries/resetting signing, but I do not think this is exposed publicly in the dashboard anymore.
Also important:
Even deleting release channels usually does not reset the signing association. The key relationship is tied to the app itself, not just uploaded APKs.
https://developers.meta.com/horizon/support/
