Hi colleen,
Probably the easiest way to do this is to edit the header cells as a "3-d range". Such a range has "depth" in that it includes other worksheets than the one that is active. To do a 3-d cell edit, select ALL the sheets you want to be changed when you edit a cell on the active sheet. To select all worksheets, simply right-click on the tab of the worksheet you want to type the heading into, then select Select All Sheets. You can of course select just certain sheets by holding down the Ctrl key as you left-click on the worksheet tabs.
Once the sheets are selected, anything you enter in a cell in the active worksheet will be entered into the corresponding cells on all the other selected sheets.
Be careful! If you forget to unselect the sheets when you make changes intended for only one sheet, you may inadvertently wipe out data in the other sheets. Of course, an Undo will usually get you out of this if you discover the problem soon enough.