DISABLING AUTOMATIC DOWNLOADS on iMAC

How do I turn off automatic downloads on my iMac? Because of this, I keep losing a free, basic but perfectly adequate Invoicing app as it auto-upgrades BUT DOESN'T WORK as it wrongly assumes I have Yosemite. I don't. I'm perfectly happy with OS X 10.9.5 (Snow Leopard/Mavericks? Can't remember).

It means I daily have to trash the updated app and get the previous version from Time Machine. Doh!!