Where can I download an old version of the Hueman theme ?

You can download a previous versions of the  Hueman theme in two places : WordPress and Github.

On WordPress.org 

For example, if you need to download v3.1.0, use this link :  https://downloads.wordpress.org/theme/hueman.3.1.0.zip

You can change the version number (3.1.0 in the example) by the one you need. 

If you don't know which version you want, the version history is available here : https://wordpress.org/themes/hueman/

On Github.com

Visit the  releases page, and download the .zip file.