It’s not possible to cancel or amend an extension once it’s been accepted. 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.
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.
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."