Flipping image (.tga) horizontaly and verticaly using C

Question

I have an assigment in C, where i have too flip a .tga image horizontaly and verticaly by doing changes to the header. I am guessing i have to do something to the height and width of the picture, but not sure what to do.

Here is a link about the TGA format: http://www.fileformat.info/format/tga/egff.htm

I am not expecting anyone to code a solution, but if anyone have a theory on how i can do it, that would be great.