Users can upgrade the iOS 7 operating system on an iPhone using the Software Update option in the Settings Menu. Apple no longer supports iOS 7 as of 2015, so the only option officially supported for iOS 7 users is to upgrade to the newer iOS 8.
Apple recommends that users back up the contents of their iPhones using iTunes or iCloud before initiating a software update to guard against the possibility of data loss from the upgrade. After backing up an iPhone, users can initiate the software upgrade by selecting the General option, then Software Update in the phone's Settings menu. Alternatively, they can use iTunes to perform an update on iPhones that have been physically connected to a computer running iTunes using the Restore iPhone button. This alternate method can be useful for users who want to wipe their iPhones or restore from an iTunes backup during the upgrade process.
Since Apple no longer supports or updates iOS 7, users should be careful when opting to update their iOS version as it is difficult or impossible to revert back to iOS 7 after the update to iOS 8. Some apps may not work or may perform poorly in iOS 8, while older iPhones may be unable to handle the demands of iOS 8 as well. While Apple officially supports iPhones as old as the iPhone 4s in iOS 8, many users with older phones report a significant degradation in performance after upgrading.