OS X Yosemite: Reinstall apps that came with your Mac
If you delete an app, you can reinstall it.
To reinstall an app that came with OS X (such as Mail or Calendar), reinstall OS X.
Reinstalling OS X doesn’t erase your information.
To reinstall iTunes, download it from the Apple Downloads website.
To reinstall Pages, Numbers, Keynote, iMovie, or GarageBand, use the Mac App Store.
If the apps came on a disc, use the disc to reinstall them instead of the Mac App Store.