You are starting with a JSON string and then you attempt to update this string which you can but if you are not saving the updated JSON then everytime the initialization takes place, the JSON will go back to where it started.
The correct way is to have a variable with the JSON data as string only.
In the screen opens or starts you convert it to object using another variable and then use the object to get or set the values.
You will still need to save the updated JSON if you want to retrieve it later.