You can do this by using roipoly function.
for ore infor see : https://www.mathworks.com/help/images/ref/roipoly.html
I am attaching sample script for it :
% read the image
input = imread (I)
%Co-ordinates of selected area(change thee according to your requirements)
r=[40 204 267 102];
c=[136 125 210 219];
BW=roipoly(input,r,c);
%replacing white area with the desired image
out=zeros(size(BW,1),size(BW,2));
out = gray .* uint8(BW); %store those pixels in output which are selected, can be done by 2 for loops also
imshow(out)
