One option is that you open all of them in the associated Office program and manually save them in the newer (docx, xlsx or pptx) format. Or follow these steps and convert all documents in one go.
Step 1: Download Migration Manager kit and extract it into a new folder – say: c:\office.
Step 2: Download and install the Office Pack – this step is required even if you have Microsoft Office 2007 already installed on your computer.
Step 3: Assuming that you extracted the Office Manager files in c:\office directory, go c:\office\tools, open ofc.ini using notepad and add the following line.
Bulk Convert Old Documents to Office 2007 Format
This refers to the folder location that holds your office files. I am pointing this to my Documents folder but it could be different on your machine.
Step 4: Open command prompt and go to c:\office\tools. There you’ll see a utility called ofc.exe – this is the Office File Converter that will convert all old Office files to the new 2007 Office document format in bulk. Run.
Immediately all old Office files in that folder (and sub-folders) will get converted to the new format and are saved in a new folder.
This utility works with Word (doc to docx), Access databases, PowerPoint (ppt to pptx), Visio diagrams, Excel (xls to xlsx) and Microsoft Project files. The conversion may however fail for password protected documents.