How do I manually change an extension from .mpg to .m2t?

Click on the filename once to select it. Click on it again. A small edit box opens. Use the backspace key, or type the new filename.

Click once....click once more....is NOT a double-click.

Or hit F2 which is rename. Or select File -> Rename

go to www.download.com
download "AF Rename Your Files" software.
try it (shareware).
very useful whe you have to change extensions for a large amount of files.
hope it helps you.

Thank you much guys, I didnt need to change my ext anyway it was my mistake that I didnt render to transport in Vegas from the beginning