Once the transactions have been posted, they cannot be deleted. You need to reverse the transaction and reassign to the correct client.
To reverse a transaction:
Open Debtors module
Navigate to the Tools menu and select the option Debtors information maintenance
Enter in the Debtor name that the original receipt was entered against, and click on the Allocate button
Check if the original receipt is allocated to any other transactions and if so, enter 0 in the cells where amounts are allocated to other transactions.
Raise another receipt for the same debtor with a negative amount.
Allocate the negative receipt against the original receipt.