If statement Matlab

I have the following code

for a = 1:38;
    for b = 1:41;
        if distance(a,b)== 0;
            distance(a,b) == 1;
        end
    end
end

Its for a 38x41 matrix. The matrix has numbers and zero values. How do I write an if statement that will convert any zero value to 1. I can't see why the above code will not work.

Regards, Jer