How to update .config file using python?

I have a .config file that is in following structure:

model {  ssd {   num_classes: 4   box_coder {   faster_rcnn_box_coder {       y_scale: 10.0       x_scale: 10.0       height_scale: 5.0       width_scale: 5.0  }}

I need to update the variable num_classes. How can it be done? Thanks in advance.