How do you exclude directories/files in managed backups?

Hi, I am using shared hosting and have a few WP site on the one hosting. So far I have not been able to backup the main site but I have been able to backup the add-on domains which each have their own directory.

I have a feeling that the automatic managed backup is trying to backup everything in the public WWW directory when I try to back up my main site, as all of the other sites are a directory in the public WWW folder.

So I only backup the relevant folders and files for my main site, how do I exclude other files/folders in the public WWW folder?