If you tried to update to iOS 8 during this evening, then there is a possibility that the Settings application will have told you that you need more available space to install the new version of the operating system. According to users who tried to update during this evening, iOS 8 requires up to 7 GB of free space in some iPad tablets to be able to make a OTA Update, in the case of iPhones, the value being somewhat lower.
If you have a terminal with 8 GB or 16 GB of storage space, then there is a good chance that this space is not available for iOS 8, and in this situation you will have to update through iTunes. On the page DOWNLOAD iOS 8 you can find download links for each version of the operating system, and using them in iTunes you will be able to update using less space than is necessary for a OTA Update to iOS 8.
