When creating an extracting rule it is important to use the right sample document, since this document plays a vital role in creating accurate extracting rules. However, sometimes extracting rules might fail extracting data from specific documents properly. Therefore, you will need to use that specific document for which extracting rule(s) failed as a sample document and test your extracting rules with that "problematic" document and find the ways of tweaking your extracting rules to tackle the exceptional cases.
In order to change the sample document for creating an extracting rule go to extracting rules by clicking on 'Extracting Rules' button of the relevant extractor. Then click on the extracting rule you want to test against a new sample document. Then, in the bottom of the page you will see a 'Change Sample Document' button. When you click it a window will popup as below. Select one of the documents from dropdown list which will be used as a sample document. Note that only last 50 uploaded documents are shown in the dropdown list.
The following video episode demonstrates changing a sample document.