Need to apply ILM policy on existing index

Hello team,
I have created one index kafka-facebook and missed to apply ILM policies.
Now all data is going into S3 bucket like: partials:kafka-facebook-2021-12-04-000035

The above index has almost 500 GB of data. can you please help me to apply new ILM policy without deleting existing data

Please don't create multiple topics on the same question - Need to apply ILM policy on existing index