In order to unconfirm a document, select the button available on the Documents screen in the menu expanded for the entire document:

Additionally, when previewing a confirmed document that has not been synchronized in the ERP system yet, it is also possible to unconfirm the document by selecting the button.

Deleting documents
To delete an unconfirmed document, select the button. The option is available on the Documents screen in the menu expanded for the entire document.
