@ushasri, If you are reading an Excel File, you can use an ConvertRecord processor, in which you define an Record Reader (CSVReader most likely) where you tell NiFi how to read the value (value separator). Next, with the help of a Record Writer (I assume CSV again or maybe Avro) you define how your data should look like, including the header. For both the Reader and the Writer you will have to define a schema, where you can do the replace of blank space with underline.
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