How to change a docker image


I pull an image I found on docker hub, I made some changes and it is working locally.

But now need to push it to docker hub. I used the following guide and I was able to push the image on my repository. The problem is that the changes I made are not in the image I just pushed. I think the reason is because I didn’t build(after making the changes) from the docker file. But given that the image that I modified is not mine I don’t know where can I find(if I can ?) the DOCKERFILE, build the image and then push it.

Thank you