How do I input a different character into a matrix full of characters?

I have generated a matrix using the following code (width and height are user input):

field = ones(width,height) .* 'O';

field = char(field);

I need to put a letter different from 'O' at several points across only one row (say the top row).

Answer 1

[the cyclist]

width = 4;
height = 5;

field = ones(width,height) .* 'O';

field = char(field);

field(1, [2 3 5]) = 'K'