至于我从

要下载旧版本的Xcode，您需要使用开发人员帐户登录

I just updated my Xcode to the newest version (7.0 7A220) and I lost the iOS 7 simulator that I had in the previous version. I still have the iOS 7 SDK and was wondering if there is way to use it on the newest version of Xcode, instead of installing another version of Xcode on my OS X.

As for my initial analysis referred from link you cant install it and use older version of Xcode version for using respective iOS simulators. To use iOS 7.1 go for Xcode 6.4 or earlier. Install Xcode 6.4 and download respective simulator from Preference.

To download older version of Xcode you need to login with developer account https://developer.apple.com/downloads/