How to restore pre-update OS from Time Machine?
If you’re looking for a certain file or folder, start by connecting the external drive that you use for Time Machine backups or by making sure that you can connect to your Time Capsule. Click the Time Machine item in the menu bar at the top of your screen (it looks like a clock with an arrow running counterclockwise around it), and choose Enter Time Machine. Here, all of your saved backups will appear in chronological order. Use the visual timeline on the right side to scroll through your backups and look for specific items or folders.

Select the file or folder you want, and press the Restore button. The file(s)/folder(s) will automatically be copied to your desktop or to the file’s original folder. This may take some time, depending on the size of the transfer.