Josh,
If this is a General Journal type of transaction, you can locate the transaction in the General Ledger Entries table and click Reverse Transaction. This will create a reversing entry and you can create a new General Journal entry to properly post. If the entry can from a Sales document, you can open say a Posted Sales Invoice and click Create Corrective Credit Memo which will create a Sales Credit Memo and you can create a replacement Sales Order to reprocess correctly.
If you are looking for some automated way of taking a bad Journal entry and inputting the correct GL Account their is no simple way just what I have written above.
Hope this helps.
Thanks,
Steve