How to convert string into json and change its value using jquery?

You can convert it with JSON.parse()

var obj = JSON.parse(state);

then you modify what you want:

obj.dataField.name = "newName";

and finally you get your string back to the state:

state = JSON.stringify(obj);

JSON.parse() can be used for converting string into JSON

var obj = JSON.parse('{ "name":"John", "age":30, "city":"New York"}');

JSON.stringify() can be used for converting JSON back to string

var myJSON = JSON.stringify(obj);

You can do JSON.parse to convert the string to json & then update the value of dataField and again do JSON.stringify

Note in the string "dataField":"{"id":52,"name":"CropSelection"}" this is wrong, It need to be "dataField":{"id":52,"name":"CropSelection"}.

let newDT = {
  "id": 100,
  "name": "test"
}

let state = '{"instanceId":"iaFMRu32kedGmOHC8umMQ","dataField":{"id":52,"name":"CropSelection"},"validation":"incomplete","required":true,"value":"","hasValue":false,"requiresSave":true}';

let k = JSON.parse(state);
k.dataField = newDT
console.log(JSON.stringify(k))

You can use the build in JSON functions. I also fixed your JSON string. You can surrounded your "dataField" value with quotes which it doesn't need. It should be {"id":52, "name":"CropSelection"} instead of "{"id":52, "name":"CropSelection"}".

var state = '{"instanceId":"iaFMRu32kedGmOHC8umMQ","dataField":{"id":52, "name":"CropSelection"},"validation":"incomplete","required":true,"value":"","hasValue":false,"requiresSave":true}';

var obj = JSON.parse(state);

obj.dataField.name = 'newName';

console.log(JSON.stringify(obj));

Posting this answer in case you posted your json incorrectly, and the value of the dataField is indeed a nested string containing json. If so, you can parse the state, and then parse the dataField. Change the parsed data field, then convert it back to strings incrementally.

var state = '{"instanceId":"iaFMRu32kedGmOHC8umMQ","dataField":"{\\"id\\":52,\\"name\\":\\"CropSelection\\"}","validation":"incomplete","required":true,"value":"","hasValue":false,"requiresSave":true}';

var parsedState = JSON.parse(state);
console.log( parsedState );
var parsedDataField = JSON.parse( parsedState.dataField );
console.log( JSON.parse( parsedState.dataField ) );

parsedDataField.name = "New Name";
parsedState.dataField = JSON.stringify( parsedDataField );
state = JSON.stringify( parsedState );
console.log( state );