How to change brightness of selected points of an image?

I know that I can change brightness of an image using following instruction:

I=(readimage);
I1 = I - 100;
imshow(I1);

But I want to know that How can I change brightness of the image just in some specified points? For example a square of the image?

[Kevin Chng]

Indexing your I will do

I([10 50],[10 50],:) = I([10 50],[10 50],:) - 100;

[Ali Purse]

May you please help me that what are the numbers?

Answer 1

[Image Analyst]

Try ginput():

uiwait(helpdlg('Click a point'));
[x, y] = ginput(1);
row = round(y);
col = round(x);
thisGrayLevel = grayImage(row, col);
grayImage(row, col) = thisGrayLevel ^2;

Beware that if your image is uint8 and the square goes beyond 255, it will clip to 255.