The below code might help.
% Read the image
img = imread('cameraman.tif');
% Extract your Region-Of-Interest (ROI)
ROI = img(50:200, 50:200); % Change these values according to your need
% Perform histogram equalization on the ROI
histEqROI = histeq(ROI);
% Replace original ROI histogram equalizated ROI
resImg = img;
resImg(50:200, 50:200) = histEqROI;
% Display results
figure
subplot(1, 2, 1)
imshow(img)
title('Original image')
subplot(1, 2, 2)
imshow(resImg)
title('ROI histogram equalizated image')
