How to fix Yosemite not to update all the time?

I have MacBook Air and I have installed Yosemite on it. Whenever I boot my Book, I get a 'Update Required' message and I end up updating my OS. I don't think that Apple releases an update each day, does it? What might be causing my Book to update Yosemite each time I boot? How to fix this or stop the update every time I boot?
