Если вдруг ещё актуально, то
тут есть ответ:
Q: Is it possible to install Xcode 10.2 on High Sierra (10.13.6)?
A: Yes it's possible. Follow these steps:
Download Xcode 10.2 via this link (you need to be signed in with your Apple Id):
https://developer.apple.com/services-account/downl... and install it
Edit Xcode.app/Contents/Info.plist and change the Minimum System Version to 10.13.6
Do the same for Xcode.app/Contents/Developer/Applications/Simulator.app/Contents/Info.plist (might require a restart of Xcode and/or Mac OS to make it open the simulator on run)
Replace Xcode.app/Contents/Developer/usr/bin/xcodebuild with the one from 10.1 (or another version you have currently installed, such as 10.0).
If there are problems with the simulator, reboot your Mac