How to reverse an ACH file, ACH batch, or ACH record?

There are several steps to edit the original ACH file:

1. if reverse the whole file, no need to change the file header record - "1" record.
2. change all the "5" records, replace the description field with "REVERSAL". ACH service class code (e.g. 200) might have to be changed depending on its value.
3. change all the "6" records to swap the transaction code for each of them. Swap transaction 22 to 27 and swap 27 to 22. (or 32 to 37 and vice-versa)
4. change all the "8" records to swap the position of the amount. If it is debit amount, change it to the position of credit amount, and vice-versa.
5. change all the "9" records with updated batch counts, entry/addenda counts and credit / debit total.
6. keep the rest of information the same as the original value.

a quick ACH format reference on web:

http://members.aol.com/getmydata/pskb/NACHA-ACH-CCD-Format.htm