To set your preferred screen saver:

From the Apple menu, choose System Preferences.
Click on the Desktop & Screen Saver button.
Click the Screen Saver tab. Available screen savers appear on the left side of the window and a preview of the selected screen saver appears on the right.
Select one of the screen savers in the left pane. To see a full-screen preview, move your cursor over the right pane and click the Preview button.

Apple Screen saver settings tab display

To set your screen saver start time:

Follow the steps above to get to the Screen Saver window shown above.

You can tell OS X when to activate the screen saver by choosing an idle time from the "Start after:" menu at the bottom of the preferences window. For example, if you don't touch your mouse, keyboard or trackpad for 20 minutes, the screen saver activates.

Apple Screen saver settings tab set time display

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