Why are you originally defining A as A = [a b c]? This isn't very common or efficient matlab usage. I would suggest starting with A = [1 2 3] and adjusting the values as needed. As for the change, see below.
% Defining A
A = [1 2 3];
% Changing A to 2 4 6
A = [2 4 6];
% or
A(1) = 2;
A(2) = 4;
A(3) = 6;
