(Blog post http://milospjanic.blogspot.com/2011/04/changing-decimal-separator-in-excel.html)
However you can do the following:
Go to Tools/Options/Language Settings/Languages
Change Locale setting to the one that has decimal separator you need, e.g. to English US if you want dot (.) to be the separator.
Leave checked Decimal separator key: Same as locale setting
And you are done.