Basically, you can search youtube for iOS7 free install without developer account. The steps are as follows:
1. download the ios7 system package
2. in iTunes, for mac, hit command key; for windows, hit Shift key, choose restore iphone, and choose your downloaded ios7
3. It will automatically install the new OS
1. If error 14 appear, try to re-download the iOS7
2. back up your iphone in the computer, it will save your contact, etc.
3. first restore your iphone to the newest OS (6.1.3 or 6.1.4), then set it as a new iphone, then do the ios7 restore
4. Using your iTunes to sync your apps (which will not included in the backup content)
5. Since it is beta version, there is some bugs. E.g. after choose share on the photo app, it will become black screen, you need to kill it in the thread. For skype, it can not go to message UI, it will directly shut down the skype app.