Below are five independent methods, but you need to try them one by one(If former method doesn’t work, you need to try the latter, until you try all 5 methods).
1. Hard Reset Your iPhone
You might like to hard reset your iPhone when it gets crashes in any situations, and it is really helpful in some cases, so recommend you can try this first when you found that your iPhone get freezes during iOS upgrade. Simultaneously hold down the Sleep/Wake button and the Home button around 10s.
2. Plug Your iPhone into a Wall Outlet
Sounds strange but the voltage power of your iPhone may be unstable, and lead to the crashed problem when update to iOS 9, so try to plug iPhone into a wall outlet, wait 10 to 15 minutes.
3. Restore Your iPhone in iTunes
If you've been backing up your iPhone, you can restore your iPhone in iTunes. First try doing it in the normal way, if that doesn't work, switch into recovery mode. When get all content and setup back onto your iPhone, you might get rid of the crashed condition from your iPhone.
4. Repair iOS System Errors
Some errors may occurred when update iPhone to iOS 9, so if the three methods above can't fix your problem, you need a third-party software like iPhone Care Pro to repair those system errors, the software can fix iOS crash, upgrade failure and any iOS stuck issues.
5. Get Help form Apple
Sometimes hardware or software picks update time to fail. And you need to get some help from an Apple retail store.
We try to offer a tutorial that you can operate steps by steps(every step is independent) to fix iPhone crashes during iOS 9 upgrade. General speaking, the methods above can fix your frozen iPhone in any cases, please try!