If you have a 2D array and want to slice out part to operate on, you can use the ;; notation in the array indices. For example, given a 3x4 array you can extract the 2x2 subarray for rows 2-3, columns 3-4 as follows.

In[1]:= a = {{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}}

Out[1]= {{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}}

In[2]:= a[[2 ;; 3, 3 ;; 4]]

Out[2]= {{7, 8}, {11, 12}}

You can use the same notation to then modify that region of the array:

In[3]:= a[[2 ;; 3, 3 ;; 4]] = {{-1, -2}, {-3, -4}}

Out[3]= {{-1, -2}, {-3, -4}}

In[4]:= a

Out[4]= {{1, 2, 3, 4}, {5, 6, -1, -2}, {9, 10, -3, -4}}