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) Click on the word "deductions" near the upper left of the home screen ->
2) Select the incorrectly recorded drive ->
3) Tap Edit ->
4) Tap Delete in the menu in the upper righthand corner
Then manually add the correct drive
1) Click on the green "add" button on the homescreen ->
2) Select "Add an expense" ->
3) Select "Add miles manually" ->
3) Enter 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!