Yosemite fail - how do I revert back to snow leopard?
Power off the Mac and then power it on holding option
Insert the Snow Leopard DVD once the grey screen is presented and choose to boot from Snow Leopard media
Wipe the Mac with Disk Utility (erase/repartition the internal drive)
Install Snow Leopard


Your Mac should have come with CD/DVD discs that allow you to reinstall the OS. Boot the system from that media (insert disc hold C while booting) and reinstall the OS. You can order a replacement from Apple or get one on the used market if you don't have yours.