(No Jailbreak Needed)
1. First of all make a backup of your device to your computer using iTunes.
2. Download latest iOS 6 (iOS 6.1.4 for iPhone 5, 6.1.3 for all other devices) and iOS 7 beta for your device from the links below.
3. By clicking shift + restore button on iTunes, restore your device to latest iOS 6 version you downloaded and SETUP AS NEW DEVICE.
4. Now, when the restore has finished and your device is setup as new, and is on the home screen, click shift + check for update button on iTunes and select the iOS 7 file you have downloaded.
5. Now, you have installed iOS 7 beta! You can restore your apps, music, photos and all your data to your device by clicking Restore backup on iTunes.
(Note1: If you want to restore to iOS 6, click shift + restore button on iTunes and select the iOS 6 file you have downloaded. Then you can again restore your personal data by clicking on restore buckup button on iTunes.
Note 2: If iTunes some times does not recognize your device at the process of iOS 7 installation, close iTunes and reopen it again.)
iOS 6: http://www.getios.com/
iOS 7 beta: http://everythingidevice.com/ios-7-beta-downloads/