问题
I accidentally upgraded my iPhone 4 on 5.1 and now I can't test on Xcode 4.2 on my snow leopard. I don't want to upgrade to lion to download xcode 4.3 because I heart many people that said it slowed down their macs and had other several problems. Is there a way to downgrade from iOS 5.1 to iOS 5.0 and be able to test again my applications on xcode 4.2 on my snow leopard? I'd be grateful if someone helps thanks.
回答1:
Xcode 4.2 can be made to work with iOS 5.1 devices.
Download Xcode 4.3 from here. Copy the 5.1 SDK and Device Support folders from the 4.3 installer into your Xcode 4.2 folder.
The former goes into Platforms/iPhoneOS.platform/Developer/SDKs and the latter into Platforms/iPhoneOS.platform/DeviceSupport
回答2:
Apple has been done signing iOS 5.0 for a while and just stopped signing iOS 5.0.1 either yesterday or the day before (I for sure know they are done though). So no you can't downgrade to any other version of iOS 5.
回答3:
If Apple is still signing 5.0 firmware installations (which is unlikely but worth a try) then user1116644's instructions will work.
Otherwise, the only way to downgrade from 5.1 is if you have an SHSH blob from one of the iOS 4 releases saved using TinyUmbrella. Then you can use that to install using the same procedure as user1116644 described, but using the TinyUmbrella TSS server to do the signing (more info here: http://thefirmwareumbrella.blogspot.com/)
Unfortunately Apple changed the way signing works for iOS 5 and SHSH blobs can no longer be used to downgrade to 5.x firmwares, only 4.3.x and below.
Edit (4-30-12): Actually since I posted this, it looks like a workaround has been developed to allow iOS 5.x downgrading as long as you saved your SHSH blob.
回答4:
Yes.
download the 5.0 firmware from felixbruns.de
put the device in DFU Mode
when you restore, hold shift and click restore a the same time.
select the firmware you downloaded
来源：https://stackoverflow.com/questions/9649313/is-there-a-way-to-downgrade-from-ios-5-1-to-ios-5-0