You cannot edit or delete a transaction whose status is Posted.
There could be a few reasons as to why a transaction's status is listed as posted.

1. If that item is cleared (checked off) in an open reconciliation statement 

To undo this, go to Accounting > Bank > Details > Reconciliation
Locate the open statement and select Details

Here you will need to uncheck (unclear) the transaction and "Leave" the reconciliation in order to remove this transaction from its posted status.

2. This item may be cleared or uncleared in a CLOSED reconciliation statement 
(the month which matches the date of this transaction)

Ex: The transaction is dated 1/12/14. This transaction will be marked as posted if the 1/31/14 statement is reconciled, regardless if the item has been cleared

To resolve, you must Open that reconciliation statement (and any others since that date) by selecting the statement and clicking Delete, then Open Reconciliation

3. You have already created a deposit slip for this transaction.

A deposit slip may be created for 2 reasons
1- You  created one yourself to lump checks together which were deposited on the same date (helps with reconciling)
2-  You received new funds and applied that one transaction to multiple matters for that client. They are then automatically grouped in a deposit slip

To undo this, go to Accounting > Bank > Details > Deposit Slips
Select related deposit slip and click either edit (to remove transaction) or delete to delete the entire slip.

Wondering what Deposit Slips are for? See HERE