If you don’t see the version you need listed in the build dropdown:

Go to Settings > Components and select + button at the bottom, and download the one you need:

If after a successful download, you still don’t see this version listed here:

It’s either one of two things:

• Try selecting earlier versions of “Device Type” before checking the “OS Version”. Some earlier OS versions are not supported on the latest device
• If that doesn’t work restar your computer. Sometimes Xcode doesn’t detect newly installed simulators.