You can't amend an extension. You will just pay the lesser amount now and mark this when you file your actual return.
Make a note of how much you pay now and when you are ready to file the actual return, go to the Extension Payments section inside TurboTax and put the correct the amount of the payment in this section.
- If online: Log into your account and click Take Me to My Return (you must click this first)
- Use the search feature in the top right-hand corner of your screen (if using desktop, use the search bar or search topics function)
- Search for "estimated tax payments" and click the link
- Scroll down to Payments with Extension
- Enter your payments