Unfortunately there's no way to edit a recorded drive! You'll need to delete the incorrectly recorded drive, and then manually add in the drive with the appropriate mileage.
First delete the old incorrect drive:
1) Select Tax Savings ->
2) View All Transactions ->
3) Select the incorrectly recorded drive ->
4) Tap Edit
5) Tap Delete
Then manually add the correct drive
1) Go the menu ->
2) Select "Add a trip manually" ->
3) Add in your actual mileage distance, date, and any notes
4) Tap Save
After you follow these steps you'll be up to date with the correct mileage!