First, export your existing keystore (Projects > Export Keystore) and save it safely somewhere offline, preferably in multiple places. Next, delete your copy in App Inventor via Projects > Delete Keystore. Confirm you really intend to do this as it is an irreversible operation (except that we are keeping a copy). Rebuild your AAB, which will generate a new keystore and sign your AAB with it. Upload this new AAB to Google.