Verision roll back

Updated Elasticsearch v6.1.3 to v6.7.1,data files can be identified automatically.Unfortunately,when i want to roll back, elasticsearch v6.1.3 can not identify the modified data files.It told me there was an illegalArgumentException: Unexpected fileld [mapping_version].
How to roll back the version correctly?Besides,I don't have more disk space to do SNAPSHOT.

Elasticsearch does not support rollbacks. The only way to roll back is to restore a snapshot taken before the upgrade.