If you've booted from a USB stick that you created using Dan's instructions, choose Disk Utility in the OS X Utilities screen and then erase the volume that currently holds Yosemite. Quit Disk Utility and then click Reinstall OS X. You Mac will install the copy of Mavericks that's on the USB stick.
Restore your files
Once your Mac has returned to Mavericks, restore any files you created under Yosemite. If you copied those files to a drive or cloud service, simply drag them to your Mac. If you used Time Machine under Yosemite, make sure the drive you backed up to is accessible to your Mac, launch Migration Assistant (found in /Applications/Utilities), and choose to restore your files from the Time Machine backup. If you initially backed up your Mavericks volume by cloning it to another bootable drive, simply boot from that drive, erase the volume that contains Yosemite, and then use your cloning utility to restore the clone back to the now-erased volume.
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