Apple presented iOS 7 last week at the WWDC 2013. If you do not want to wait until fall, here is a HOW TO install iOS 7, even if you do not have an Apple Developer Account:
1) Download iOS 7.
Search for iOS 7 Beta on Google and you will find a pre-release version of iOS 7 for the respective device.
Here is one possible source. (No liability on my end.)
2) Make a Backup!!!
There is always a chance something goes wrong or your phone does not function as smooth with the new operations system. To avoid a catastrophe, save your data on your computer via iTunes.
2) Restore your iPhone.
Connect your iPhone to iTunes via USB and klick on Restore. This will clear out your phone and put it into default mode.
3) Install iOS 7.
After your phone is restored and running again, set it up as new and connect it to iTunes again. Search for updates by klicking on the button while you hold the SHIFT (Windows) or ALT (Mac) key. This allows you to manually search for the iOS you downloaded earlier. Select the iOS 7 beta file (.ipsw).
4) Wait. Restart.
Your iPhone should restart automatically after iOS 7 is installed. Enjoy!
I received positive and negative feedback. In one case, the iOS would not run at all on an iPhone 4S, however, it works fine on mine. Just in case you are not pleased with the beta version, make use of the recovery mode and restore your iPhone with your iOS 6 Backup.