I recently have update the XCode to the version 7.0, and because of this my older version simulator ios 7.1 should be reinstalled again, but i don't konw how to do this .
how can i install the ios7.1 simulator for xcode7
I don't think the iOS 7.1 simulators work with Xcode 7. Googling "xcode 7 ios 7 simulator", everything seems to indicate it can't be done. I guess you could install Xcode 6.4 (drag to Desktop instead of Applications, rename, drag to applications - this prevents overwriting Xcode 7) and run it from there, but if you are using Swift that won't work because of the different versions of Swift.
I have this exact same problem. Apple does this every time they update Xcode by a major version! In the past I would always just keep the older Xcode version around so I could open my projects and test them on the older simulators, but it wont even build in Xcode 6.4 anymore.
Hey Apple, how are we suppose to support older iOS versions without being able to test on them??
Apple's position is that we need to test on actual devices that have not been upgraded. Since downgrading devices is not possible it's also not possible (well not feasible for small time developers at least) to test on all device and OS version combinations. In practice this doesn't turn out to be that big a deal as the upgrade rate of users is so high.
Myself I keep an old iPhone 3GS for iOS 6 and an iPhone 4 for iOS 7. Each was well under $100 used. iPods should be even cheaper. iPads hold their value better so it's more expensive if you need those.