How to process a subset of a matrix

I have a nxm matrix and I want to divide the first R rows by a certain number. For example,

    2   4    6   8
   10   12  14  16
    1   3    5   7
    6   7   13   9

I want to divide the first two rows by 2 to get the modified matrix as

    1   2    3   4
    5   6    7   8
    1   3    5   7
    6   7   13   9

I know its simple but how to do it?

Answer 1

[David Fletcher]

matrixName(2,:)=matrixName(2,:)/2