Matlab code without for loop

Image contains 4 pixels . 0-background, 1,2 and 3 freground pixels.Can anyone help me how to improve the following code for computational efficiency? Thanks

[m n]=size(image);
for p=1:m
    for q=1:n
        if image(p,q)==1
            image(p,q)=255;
        else
            image(p,q)=0;
        end
    end
end

Answer 1

[jgg]

I think this should work:

 i = image('file.jpg')%load in whatever image it is
 i(i == 1) = 255;
 i(i ~= 255) = 0;