How to stop the up grade from Maverick to Yosemite

If the Yosemite installer has been fully downloaded to your Mac, it will start automatically and then wait for you to tell it to proceed; it doesn't simply take over and complete the installation. The Installer should be in your Applications folder and is called "Install OS X Yosemite." You can simply quit the installer, and it will then not proceed.

I'd advise you to archive the installer in case you change your mind. I'm sure you've seen posts here from people who wished they could install Mavericks but can no longer download it.

I don't believe you can undo it; however, there is an easy way: simply finish the download. Downloading the OS does not mean it is installed. Keep an eye on the download - when it is finished, you will get a new window; it is the actual installer. Quit the Installer (Command + Q or File menu >Quit). You will find the installer in your Applications folder - you can copy it to some other safe place to keep for the future or delete it. If you want to use it later, simply put it back into the applications folder and double click to start the process.

After it finishes downloading open launchpad press mouse button until apps giggle just like on iPhone iPad and then click on the x that appears. Download deleted.