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
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