If you accidentally delete a transaction, you can restore it. To restore a transaction that was deleted:
On the mobile app:
Tap on the Expense or Income tab and go the Reviewed section
Tap the gray bar with the year
Select Archived
Tap on the transaction
Click Restore
On the web app:
Go to the Income or Expenses page
Select Archived from the Type drop-down
Click on the transaction
Click Restore at the bottom