A slight variation on Richards answer but readTree can take a string so you can simplify it to:

ObjectMapper mapper = new ObjectMapper();
JsonNode actualObj = mapper.readTree("{\"k1\":\"v1\"}");

import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.node.ObjectNode;

// Read JSON file into a JsonNode object
String jsonStr = FileUtils.readFileToString(/* file location */, 
StandardCharsets.UTF_8);
JsonNode rootNode = new ObjectMapper().readTree(jsonStr);

// Find the field and update the value
ObjectNode target = (ObjectNode) 
rootNode.path("parent").path("child1").path("field");
target.set("innerField", new ObjectMapper().valueToTree("somevalue"));

// Convert the updated JsonNode to a string
String updatedJsonStr = new ObjectMapper().writeValueAsString(rootNode);
LOG.info("New JSON: {}", updatedJsonStr);

First you read the JSON file into a JsonNode object using the ObjectMapper. Then we use the path() method to navigate to the field we want to update and cast it to an ObjectNode, which is a mutable implementation of JsonNode. We use the set() method to update the value of the innerField property.

Finally, we use the writeValueAsString() method of the ObjectMapper to convert the updated JsonNode object back to a JSON string.

Provided that you have already read this object into JsonNode, you can do it like this:

String content = jsonNode.get("data").textValue();

UPD: since you're using a streaming parser, this example on Jackson usage might help.

UPD: the method name is now textValue() - docs