Well you have two options:
1)Straightforward, download Xcode 9 and you are good to go.
2) You need to put DeveloperDiskImage
of iOS 11 in
/Applications/Xcode8.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport
and you can find this diskImage in Xcode9 at same path in case you want to continue with XCode8
Path of iOS 11 Disk Image:
~/Applications/Xcode9.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/11.0
NOTE: Xcode 9.0 requires a Mac running macOS Sierra 10.12.4 or later.