We should note 1st of all that you can update your iOS device through a Mac with iTunes without having much extra space. That might be the quickest/easiest way to go
Today iOS 8 came out. There was a massive rush to try to install and download it. The one thing that we were not prepared for or made aware of was the need to have up to 4.7-6.9 GB of space available in order to download and install iOS 8. The reason you needed 4.7-6.9 available is that the 1.1-1.3 GB download of iOS 8 also needs to move a lot of files around during installation.
If you are like us, you might not have that much space needed on your iOS device. We’ve spent the last year downloading music, videos and apps to the point where our iPads tell us to stop. So now we need to delete some of our stuff.
Once you free up your space on your device, and back up your device, then you will be officially ready to download iOS 8. In this how-to I will discuss the two different methods to free up storage space off the device, either from the device itself, or managing the device while being connected to the computer through iTunes…
- How-to: Shave some GBs off of your iOS device so you can update to iOS 8 (9to5mac.com)
- How to Download iOS 8 Without Deleting Any of Your Data (mashable.com)
- How to get iOS 8 and keep your photos (stuff.co.nz)
- Users frustrated by Apple iOS update (bbc.co.uk)
- Out of space to install iOS 8? Use iTunes instead! (imore.com)
- Apple releases iOS 8 for download (cultofmac.com)
- How to Install iOS 8, A Quick and Easy Guide (technobuffalo.com)
- iOS 8 Problems: Failed Updates, Cannot Restore (gottabemobile.com)
- iOS 8 update may be too big for your iPhone or iPad (theverge.com)