I plumped for Yosemite too soon!
Q At WWDC 2014, Apple released OS X 10.10 Yosemite for developers to Beta test (I’m a registered app developer), so of course, I went and downloaded the update. However, I’ve found lots of bugs in it, and want to downgrade. My Mac won’t let me download Mavericks on the Store because I have a newer OS version. Dan A You should be able to download OS X versions back as far as Lion from the Purchases tab in the App Store. But instead of running the installer, you need to use Internet Recovery mode.
The ordinary Recovery mode that boots from the hidden partition on your local hard disc simply reinstalls a clean copy of the last version of OS X you installed – Yosemite in your case. But Internet Recovery reverts to the version that originally came with your Mac. Before you do this, though, make sure you have a full backup, ideally using something like Carbon Copy Cloner or SuperDuper!, rather than just Time Machine.
Now reboot with ç+å+R held down. You’re prompted to connect to your Wi-Fi network and it downloads the installer and, eventually, OS X itself. After that you can go back to the App Store and download the upgrade to Mavericks, and then restore your data from backup.
It’s probably best to let the developers find all the bugs before you upgrade to OS X Yosemite.