Easy answer, don't! Never ever change a user's selected environment settings through your programs. This is one of the number one ways to get in big trouble from your users, and mess up any number of poorly written apps on the user's computer. If you want your numbers in a certain format use the FormatNumber function, the CInt/CLng/CSng/CDbl functions, and MaskEditBoxes.

You can change the region settings through API calls, but trust me, you shouldn't take that route.