How do you turn on "show pre-release updates" in yosemite?

I accidentally turn off "Show Pre-Release Updates" settings in the system preferences. Change option does not appear in the relevant section.

What is your suggestion?

@Mark
sorry I can't comment your question directly, since I'd need 50 reputation to do so.

For your question: Wen you quit your subscription for the seeds you basically uninstall a program on your Mac. You can re-download that program directly from Apple and install it. Since the Beta-Progamm is linked to your Apple-ID you have to log in with it. Once you downloaded that file (YosemiteBetaAccessUtility.dmg) you simply install it and you have access for the Pre-Release Updates again. Just open Appstore and install the updates.

I found the file with the following steps:

1. Go to appleseed.apple.com
2. Click "first steps with OS X Yosemite Beta"
3. Scroll down the site till you find the button where you can download the Beta Access Utility.
4. Download and install
5. Restart if needed and it's finished

The option you are referring to was listed as "Your computer is set to receive pre-release Software Update seeds" with a button labeled Change next to it. By clicking Change, you could opt into or out of the OS X Public Beta Program.

However, Yosemite is no longer in beta, and therefore the option was removed from the final release.

Edit: This information is out of date as it seems the OS X Yosemite Public Beta has been reopened.