import json

Define a JSON string

json_str = '{"name": 'John', "age": 20}'

Convert the JSON string to a dictionary

data = json.loads(json_str)

Modify the 'age' field in the dictionary

data['age'] = 25

Convert the dictionary back to a JSON string

new_json_str = json.dumps(data)

Print the updated JSON string

print(new_json_str)

Output:

{'name': 'John', 'age': 25}