how do i change the orientation of the centre square of a rubiks cube?

Take two sides, one of which is in the wrong orientation. Turn the side with the wrong orientation in the direction that takes it towards the correct orientation, then turn the other side in the same direction that it would go if the two sides were gears that are meshed. Repeat this set of two moves until the cube is solved again (I don't remember how many turns it is; it is quite a few, but is reasonable). If I remember correctly, both sides you chose will have rotated by 90 degrees. A wise choice of which two sides to use will eventually solve the cube if you have not tampered with it.

Edit: The answers claiming you have to start over are simply wrong. I know for certain that a simple algorithm like the one above exists, and I believe that this is the one that I figured out a couple years ago for the same challenge.

Edit 2: If you do the above move 63 times (a total of 126 moves) it will come full-circle and the cube will once again be solved, but the center squares of the two sides you use will have moved 90 degrees in the direction opposite the direction you have turned them 63 times (since the remainder of 63/4 is 3, and 3 turns in one direction equals 1 turn in the other direction on a Rubik's cube)

You use variations of the 4-box and the 6-box pattern to solve these. That is, you make the cube into one of these patterns, then solve that pattern back to plain again. Every time you do, you re-orient the middle squares.

Eventually you get them all facing the correct way.

There are books on this subject to give you the exact moves needed to do this faster, but I think you can find the correct moves on your own with a little twiddling.

You solved it incorrectly. The middle square of a Rubiks cube can never be never be reoriented. You must build the picture based on the center piece. On a normal cube (colors, not pictures), you move all the yellow pieces to the side with a yellow center, same for all the other colors.

Essentially, you have to start over.

You can't change the centre square. It's stuck there, so you have to change the surrounding squares so it can make sense with the centre square there.

That is, the surrounding squares are all affected by the centre square.

you can't change the orientation of the centers. those are the ones that are fixed. Instead, you must solve for the centres. That is, you must change ALL the squeares from one side to the other.