How do I overwrite/correct a federal extension amount

You can clear it and start over.   If using TurboTax Online:

1. Login to TurboTax and select Continue
2. Select Tax Tools 
3. Select Clear & Start Over.

 

Refer to the TurboTax article How do I clear and start over?

 

If you are using TurboTax Desktop, start a new return by selecting File and New Return.