You need to install cancel_entry module, then (with Technical Features enabled) allow cancelling entries for the journal type under Accounting--> Configuration-->Journals-->Journals.

Now you will be able to cancel your accounting entries and edit them, then post again..%

Hi Germain !!! How will I manage to modify a confirm entry........???

Generally In Accounting Standard it is not a good practice to modify the confirm entry .. the standard way of doing this is make a reverse entry of corresponding entry and then re-create a desired entry.

But here in OpenERP it is also given without creating a reverse entry just cancel the confirm entry say ----invoice customer/supplier invoice which has been approved, then in OpenERP account entry done as when you just approve the Invoice, and then you are not able to do change, but if you still looking for modification of this approved Invoice , then and and settings >> module >> module go and look look " account_cancel " module and install it. After installing this module move in ------Accounting >> Configuration >>>Financial Accounting >> Journal >> then select the corresponding Journal for which you would like to cancel /modify entry (ex for supplier invoice ---Purchase journal , for customer invoice -- Sale Journal and likewise ...) then look for other configuration in Journal Screen .... and Allow Cancelling Entries and tick it here make "True", Now you open your relevant screen where you want to change/modify entry there is a button "Cancel " came now click on that then it will be in cancel state and then again "set as Draft" button will appear click on that and now you are able to make edit the relevant entry .... and again more important thing you will get No. (ref no.) Hope it will help ....if yes then :D ...