MATLAB newbie: how do I change an element of a string array?

[Antst]

Hi everyone:

I have an array of strings, for example:

header = {'Fred', 'Tom'}

When I do:
header(1)

I get 'Fred'

I want to do (for example):
header(1) = 'Jill'

But I get this error:
??? Conversion to cell from char is not possible.

In other words, I want to be able to change elements. Could anyone explain how to do this? Thanks a lot.