How to replace the entries of a matrix by another matrix?

Question

I have a two matrix mm and nn. Matrix mm is of dimension 8 cross 8 and nn is of dimensions 4 cross 4. I want to replace the first four rows and columns in mm by nn. How to do this?

mm = ConstantArray[q, {8, 8}]
nn = ConstantArray[r, {4, 4}]

Answer 1

you can use Table and make any changes you like

Table[mm[[j, k]] = nn[[j, k]], {j, 4}, {k, 4}]   

{{r,r,r,r,q,q,q,q},{r,r,r,r,q,q,q,q},{r,r,r,r,q,q,q,q},{r,r,r,r,q,q,q,q},{q,q,q,q,q,q,q,q},{q,q,q,q,q,q,q,q},{q,q,q,q,q,q,q,q},{q,q,q,q,q,q,q,q}}