I guess you haven't had an answer yet because "Gibbs Ringing" is quite a technical and specialised area. However, the substance of your question has a very simple answer.
If your image array is img, then you can make a new array that is the same apart from having every value exactly equal to 70 changed to 255 like this:
img_new = img;
img_new(img == 70) = 255;
A couple of extra points, if I may.
- It's a bit surprising that you are asking a very elementary programming question when you are hoping to do something as potentially complex as simulate MRI. You may need to do a crash course on programming in MATLAB if you're going to make progress at all. There are lots of resources - search Answers for how to get started in MATLAB for image processing.
- Increasing the size of the contrast won't create Gibbs artefacts if they weren't there previously, though it might possibly make them more visible in the result. More important is to think about whether you need any frequency-domain operations to make this phenomenon significant.