From the settings menu, select Developer options, scroll down to Select runtime, and choose ART. After rebooting, your device will begin to optimize your apps for the new runtime. This should take a few minutes, but when complete you will notice a considerable performance boost on most devices.
I should warn you, though, that some (and I mean very few) apps may be incompatible with the ART runtime. If one of the apps you use is unable to open or you are having trouble receiving a software update, simply switch back to Dalvik mode using the same process.