You can not refile an extension. Only tax returns that have already been received by the
government can be amended. An extension is just a tax form; Form 4868 to
be exact.
If you e-filed your extension and it gets rejected, you can
make your changes before you resubmit it or mail it in. However, once
it's been mailed or accepted, it can no longer be changed.
Don't worry if you think you may have over- or underestimated
your tax on the extension; the government doesn't expect you to get it
right down to the dollar. In fact, the IRS instructions on Form 4868
say:
Make your estimate as accurate as you can with the
information you have. If we later find that the estimate was not
reasonable, the extension will be null and void.