1. Launch any OpenOffice or LibreOffice application such as Writer.
  2. Click on Tools and then Options.
  3. Expand the Load/Save section in the left pane by clicking the + (plus sign).
  4. Click on General under the Load/Save section.
  5. In the area labeled as "Default file format" near the bottom of the dialog window, select the type of document you want, for example, Text Document for Writer files, in the Document Type drop-down list.
  6. To the right dropdown list labeled as "Always save as", change or select the format as "Microsoft Word 2007-2013 XML".
  7. Repeat the above 2 steps for other documents such as spreadsheets or presentations that you want to save in Microsoft Office file format, and select the appropriate default save as format for them.
  8. Click OK to save the settings.
  9. From now on, whenever you want to save a document, the default format will be .docx, .xlsx or .pptx and other Microsoft Office file formats and extensions.