1. Click the File | Save As command.
2. Next to the Save as type, click the existing option and select the desired format from the list. 

 
Select the desired format from the Save as type list.

3.  After clicking Save, the file is saved in the format selected.

Note that new features may not translate to older versions. So, some items may be displayed differently when saving back to an older version.

Updated July 22, 2019