I want to change the values on a graph, but I don't have the initial data, means I have just the graph. Any idea?
If you have the image as a graphic file (.JPG, .PNG, .GIF, etc.) or a hardcopy that you could scan to produce a graphic file, there is a wide variety of graph digitising software packages that you could use to retrieve the original data points, which you could then re-plot any way you liked.
There is a Wikipedia article that provides references to a number of packages, another list of software alternatives for the same task and a CNET review of still another package that carries out this task.
Of the various packages that I've used over time to do this, the one I used most recently was WinDig though it is getting pretty old -- it is still a 16-bit Windows application -- so I wouldn't necessarily recommend that one.