I need to change a line in requirements.txt and alter the behavior of an existing docker image

I have built a docker image. I want to change the python SimpleITK library version to 2.0.2 (right now it is 2.1.1) for my docker image, by inserting a line in requirements.txt file. I want to alter the behaviour of my docker image like this without building the image from scratch. Is this possible ?

Please let me know.