{
    "Usernames": {
        "Username": [
            {
                "Password": "Password123"
            }
        ]
    }
}

jsonloads = json.loads(open('json.json').read()) #Load the json

username = input("Enter your username: ")
for i in jsonloads["Usernames"]:
    if i == username:
        pass = input("New password: ")
        jsonloads["Usernames"][i][0]["Password"] = pass
        jsonFile = open("json.json", "w+")
        jsonFile.write(json.dumps(jsonloads, indent=4))
        jsonFile.close()
        break
else:
    print("You aren't in the database. ")
    user = input()
    pass = input("Password: ")
    item = {"Password":pass}
    jsonloads["Usernames"].update({user: item})
    with open('json.json','w') as f:
        f.write(json.dumps(jsonloads, indent=4))