However, the users need not worry of content loss. All the user contents are stored in a compressed file while downloading iOS 8 and when the download is complete, the device starts unpacking the compressed file.  
Though, the device needs enough storage to handle both the compressed file and all its uncompressed contents as the system reboots.

The users can also install iOS 8 by connecting the device with computer and syncing with iTunes. Theprocess
The iOS 8 mobile operating system will be pre-installed on iPhone 6 and iPhone 6 Plus which was unveiled at Apple’s annual developers conference in San Francisco in June.
The new operating software is tailored to deliver a seamless experience for users flitting between Apple mobile gadgets and popular Macintosh computers.
Alongside operating system updates, the California company unveiled HealthKit software to manage personal healthcare and HomeKit for home appliances. They are integrated into the latest system update.
HealthKit will provide a secure haven for data collected by devices such as fitness bands that track activity and sleep.
Apple also synched iOS 8 to HomeKit software that could let iPhones or iPads be used as a centralized control for Internet-linked gadgets such as door locks, lights, thermostats and security systems.
Apple came up with a common network protocol, so not only can an iPhone be used to open smart locks, but virtual assistant Siri can dim lights and lower thermostat settings when told, "Get ready for bed."
Apple has also beefed up graphics and speed capabilities for games, which are consistently among the hottest applications on its mobile devices.
Capabilities woven into operating software include one called "Continuity," which lets tasks started on one Apple gadget be automatically handed off to another one nearby.
Messages or calls can also be handed off between devices. Apple last week announced that it is adding a smartwatch to its growing array of devices that will work together. The Apple Watch will not be released until early next year.