How can I move a 'gridbox' 1 unit of Z axis up or down?

Hi guys,

I want the 'gridbox' between x = -1 to -1.5 and y = -1 to -1.5 raised by 2 units on the z axis? This will make just that one gridbox higher than the rest of the surface.

Here's the sample:

x = [-2:.5:2];
y = [-2:.5:2];
Test = @(x,y)x + y;
[X,Y] = meshgrid(x,y);
Z1 = Test(X,Y);
s1 = surf(X,Y,Z1);