If a transaction has been entered in error - depending on the type of correction you need to make - you can 

• Re-assign the transaction to the correct constituent - this is done by finding the transaction on the constituents timeline, opening it and using Reassign from the Transaction manage menu - see example below
• Remove a transaction entirely you can use 'Delete' from the transaction manage menu 
• Enter a refund transaction you can create a payment method of 'refund' and enter a new 'refund' transaction with a negative amount