Step-by-step Guide To Clean Install iOS 8.0/8.0.2
Step 1: Download iTunes 11.4 as it has full support for iOS 8.
Step 2: Backup your device via iCloud or iTunes. It is imperative you do this before you start installing iOS 8 on your iPhone 5s.
Step 3: Download iOS 8.0 for your device; click here to download iOS 8.0 for iPhone 5s GSM, and click here to download the firmware for the CDMA model.
Step 4: If you want the iOS 8.0.2, here are the links: GSM Model, CDMA Model.
Step 5: Launch iTunes on your computer.
Step 6: If you're using a PC, press the Shift key and click the Restore iPhone button on iTunes. For Mac users, press the Alt key and click the same button.
Step 7: iTunes will now prompt you to choose the .IPSW file you want to install on your iPhone 5s.
Step 8: Navigate through the directory and select the file.
Step 9: Wait until iTunes is finished installing the new update. This process will take few to several minutes to complete so be patient.
Step 10: Once installation is finished, your iPhone 5s will now be running iOS 8.0 / 8.0.2.
For iPhone 5s owners who are currently running iOS 7.x, you can also receive a notification about the availability of the OTA update. If you decided to take the route in updating your phone, still don't forget to backup all your data.