Changing The First Character To Upper Case?

this is what im trying to do:

Code:
var field01 = document.getElementById("Field01").value;
field01[0] = field01[0].toUpperCase();

it doesnt work, the first letter of the field stays lower case BUT when I do this:

Code:
alert (field01[0].toUpperCase());

then it converts the first character to uppercase, well only in the alert box, the actual one still unchanged.

so i figure thats because it needs to be triggered somehow, it wont work by assigning it. i cant use it as return. are there any trigger methods that would just let me capitalize the first letter without any pop ups or anything?