Change in matrix without re-writing

So if I have a 2x4 matrix, how do I change a number in that matrix without having to re-write the entire matrix?

 M = [ 1 3 5 2
      -1 0 1 7];

(M is one big matrix, I'm sorry but I'm quite new here) If I wanted to change the 7 to a 8 for instance, what would be the fastest way?

Thanks

Answer 1

[Geoff Hayes]

Rand - since the 7 is in the fourth column of the second row, then the easiest way to change this to an 8 is as follows

 M(2,4) = 8;

[Rand Renwar]

Thank you very much!

Answer 2

[Steven Lord]

Indexing.

A = zeros(5, 5)
A(3, 4) = 7
A(10) = 2
A(A > 5) = 5