We have release 8.1.0 so you are far behind the supported versions.
With the recent version, you could use field aliases which has an immediate effect but the old field still exists.
If you really want to remove, there is no other way than reindexing.
I'd recommend to do a reindex from remote and reindex your whole data from 5.6 to 8.1 and using an ingest pipeline using the rename-processor.
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