The ExternalStore class (and the new ExternalStoreMedium class) has methods for storing new data, but apparently, there are no methods for updating or deleting records. This came up in T246539#5942051.
Perhaps it's not too common to delete or update records in ES, given its current usage, but I wonder if there are other reasons why these operations are currently impossible. If not, we should consider implementing them (and I can do that).