How to set field value of running class and not an instance with Java

I was able to loop through the fields of a class and grab private fields values using reflection. However I'm not able to set field values and the only solutions I can find is using newInstance() which seems to affect the value of a new instance of the class.

How can I set the value of a field in a running object Class?

Here is my compile and run error free code that still doesn't set the value of current running class I want to target.

Field[] allFields = c.getDeclaredFields();
  for (Field field : allFields) {

      field.setAccessible(true);

      String fieldName = field.getName();

      if (fieldName == "theNameOfFieldIneed"){

          try {
              Object objectInstance = a.newInstance();

              field.setInt( objectInstance, 1000);

          } catch (IllegalArgumentException e) {
              e.printStackTrace();
          } catch (IllegalAccessException e) {
              e.printStackTrace();
          } catch (InstantiationException e) {
              e.printStackTrace();
          }

      }