Post your billing to the wrong month in extendedReach? Don't worry! You can easily change the posted month by going to Organization Billing > Posting: Posted by Batch > click on "Recreated Posted Batch File".
A pop-up will appear where you can enter in the batch number. Enter in the batch number of the batch you'd like to change the posted month of. Only one batch can be updated at a time, so if there are multiple batches, repeat the process.
Click on the checkbox next to "Change Posted Month" then use the drop-downs to select the appropriate month and/or year.
Once you click on submit, the file will re-generate and the date changes made will be reflected.