If you accidentally delete a transaction, you can restore it. To restore a transaction that was deleted:
On the mobile app:
Tap on the Income or Expenses tab and tap "Reviewed" at the top
Tap the gray bar that displays the current year
Select "Archived" from the drop down menu
Tap on the transaction
Tap "Restore" at the bottom
On the web app:
Go to the Income or Expenses page
Click the "Type" filter and select "Archived"
Click on the transaction
Click "Restore" at the bottom