You will need to change the mapping for the field and then re-index the data - you cannot change a mapping for a field after you've indexed data into it.
If you are unfamiliar with the concept of mappings within Elasticsearch or the Put mappings API, I'd start reading here:
http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/mapping-intro.html
and here:
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/indices-put-mapping.html
Assuming you wanted to created a field as binary, you'd do something like this:
$ curl -XPUT 'http://localhost:9200/yourindex/yourtype/_mapping' -d '
{
"yourtype" : {
"properties" : {
"yourbigstring" : {"type" : "binary", "store" : true }
}
}
}
'
and if you want to leave it as a string but not analyzed:
$ curl -XPUT 'http://localhost:9200/yourindex/yourtype/_mapping' -d '
{
"yourtype" : {
"properties" : {
"yourbigstring" : {"type" : "string", "store" : true, "index" : "not_analyzed"}
}
}
}
'