Hi Vikas,
with a few exceptions stated here, the mapping for existing fields cannot be updated. Also, starting with 2.0 it is no longer possible to delete a type mapping. To change a mapping you have to create a new index with the new mapping and reindex your data to it. For older versions, this blog post describes the process. Also note there is work in progress to simplify this reindexing procedure using a dedicated API for this, see this Github issue for details.