One of the most popular problems that come upon you when you are trying to update your iPhone, iPad and iPod Touch to iOS 8.3 must be insufficient space. If you are updating from iOS 7 or below, it requires about 5 GB free space, depending on your iOS device model. If you are updating from iOS 8.2, the required space would be much smaller. You can firstly check how much storage space available on your device now by following the route: Settings > General > Software Update.
Below are the solutions for room making for the iOS updating.
First: Delete Photos, videos and voice memos which take much space on your device
Delete the photos, messages, videos, voice memos which you want any more. If you just want to delete the photos, messages, and voice memos etc temporarily and then restore them to your iPhone, iPad and iPod Touch when the updating finishes, you can sync your device to iTunes/iCloud before the updating. So you can make room by deleting the photos, videos, messages, message attachments, voice memos etc from your device.
iTunes/iCloud is not as trust-worthy as you may expect, there are always something wrong with no apparent reason. Worse still, it is always risky to restore from iTunes/iCloud backup file as data overriding happens from time to time. If you want a relatively clean iPhone, iPad and iPod Touch after the iOS 8.3 update, instead of recovering everything back to your device, you may try iFonebox, which is professional iOS data recovery software and capable of recovering Contacts, Notes, Reminders, Messages, Messages Attachments, Photos and Voice Memos etc from iPhone, iPad and iPod Touch as well as from iTunes and iCloud backup files.
So the best way to safeguard your data on the iPhone, iPad and iPod Touch is to extract the important data and transfer to your PC for a physical backup. This is iFonebox can perform much better than iTunes/iCloud in terms of data securing. Check out our guide list about how to extract and transfer data to PC with iFonebox
Second: Delete the Apps and Games
Focus on deleting those apps and games that occupy too much space, or that you don't use that much, even ones that can be easily replaced once the iOS 8.3 update is finished. Avoid deleting apps that may contain your personal data or projects.
Notes: Before deleting them, you'd better transfer apps & games with process and data first.
Third: Delete Junk/Temporary Files, Log Files, Cache and Cookies
There's a ton of junk files that mostly are not needed but is still using up a lot of space on your devices, and they can't be cleared manually. You can search and locate some iOS cleaning tool, to delete all that junk files including useless and unnecessary temporary files, log files, browsing/app cache, cookies, bad/empty/corrupt files and other unwanted files on your devices, helping you to reclaim up 40% - 60% storage space, a big deal to the 8GB/16GB devices.