In order to change or update the Tab Name that displays when someone opens a Document Center document, you must change the Title property of the PDF.
In order to update that title, you will need either the original document or you will want to download the document from the site.
If you have the original document, you can use the appropriate software (Word, Excel, etc.) to update the Title field in the document’s Properties. If you only have the PDF, you will need a PDF editor to update the Title field in the document’s Properties. You will then replace the incorrect document with the updated version in the Document Center.