UpdraftPlus backup mail site only

I use a support subdomain to make all WordPress changes before migrating them to my main site. All of my UpdraftPlus plugin settings in this support subdomain are migrated to may main site whenever the main site is updated, at least monthly. If I set UpdraftPlus in my support subdomain to backup, for example fortnightly, then my main site and my subdomain will both be backed up at the same time on this basis.

How can I set up the UdraftPlus plugin to only backup my main site, without having to logon to the main site and changing the UpdraftPlus settings there? I have previously resisted touching my main site during normal operation.