How to change the huge amount of data formats of my index pattern at once?

I want to change the data format from default to string.
However it is too many to change from the UI.

index_pattern

Is Reindex API able to use or other Restful API to do so?

Ideally, you should be using a defined mapping in Elasticsearch for the index so that Kibana reads the formats correctly.

So you can create the explicit mapping, and then reindex.