mirth file reader to read fhir-json content and replace few string

I want read file through mirth file reader which has fHIR-Json. I need to replace the string values and again write it in to the same file. How to do that.

e.g

{"resourceType":"Bundle","id":"66bebc03-3480-4278-9f26-f9fd72478dcd","type":"searchset",

I want to replace "resourceType" : "Patient" "type":"transaction" all other contents are same And write it to the same file.

After writing the file I have post it to another fhir server.

when I tried to read file its give me error

SyntaxError: Expected end of stream at char 33880

So far I couldnt solve it.