Apply category to multiple files

I have hundreds of files in my media library and I would like to categorize them in an easy way.

My question: How can I assign multiple files to the same category with few clicks?

I have spent quite some time trying but the best I can is to go in each file and manually select the category, which is not feasible taking into account the number of files at stake.

Thanks a lot in advance for your help.

My server runs WordPress 3.8.1 and Media Library Assistant 1.70.

https://www.ads-software.com/plugins/media-library-assistant/