how to set pixel become white pixel

i have an image, i want to set row 0 until row 20 become all white pixel in that image. here i attach my illustration pic for your understanding. help me please http://i1259.photobucket.com/albums/ii548/msaiful89/hmm.png

Answer 1

[Walter Roberson]

   YourImage(1:21,:,:) = 255;

Replace the 255 with 1 if your image is floating point.