Turning off update email reminders

Hi
I run at least 20 WP blogs. Some are up to date and others are running older versions because the themes I have will break if I update to new WP versions.

I want to know how to turn off these annoying new spammy emails that say “[Blogname] WordPress 3.9 is available. Please update!” Once was a bit much but they keep happening and it’s driving me crazy. I can’t find a way to turn them off in admin.

When you’ve got 20 blogs, this is seriously annoying. I don’t auto update my blogs due to various compatibility issues, especially when there’s a big update like 3.9 it pays to hang back and wait for the bugs to emerge.

So please please… how to turn off the emails?