View, edit, and share .dwg files

There are several ways to view, edit, and share .dwg files, the native file format for AutoCAD data files, without using AutoCAD software:

• A360 Viewer—View designs in more than 50 file formats, including .dwg, right in your browser—no downloads or plug-ins needed.
• DWG TrueView™: This desktop software enables you to view .dwg files and includes DWG TrueConvert™ software, which lets you convert newer
  .dwg files to work with older versions of AutoCAD.
• AutoCAD 360: free* app lets you view, create, edit, and share CAD drawings using a web browser or mobile device.**

The source of this infromation is: http://www.autodesk.com/products/dwg/viewers 

How to convert a dwg file to another version of AutoCAD

Simple:

1. Visit this link:http://www.autodesk.com/products/dwg/viewers
2. Download the viewer
3. Open your dwg file
4. And convert it to the desired version of AutoCAD.