When you have two versions of an Office program on the same PC, such as Word 2013 and Word 2010, you won’t be able to set the older program as the default in Default Programs in Control Panel. Only the newer version of Office will show in the list of programs.
Because you can’t set the default Office program, you won’t be able to choose which version of Office you’d like to use when you open a file.
To get around this problem, start the Office program you’d like to use first, and then open the file (File > Open) in the program. For example, if you’d like to open an older Word document with Word 2010, you’d need to start Word 2010 and then open the document in Word.