Xcode has a project setting for 'iOS Deployment', where you can choose that same (Xcode max) or lower iOS version that app can support. Of course you'll need a device with that lower IOS in order to use an older Xcode. Details may be involved that require certain app specifics, tho, so be prepared to meet them. The El Capitan install wiped out my Xcode install, so I had to search for it in the App Store and reinstall. I now believe that I deleted Xcode awhile ago and forgot, hence the reinstall. I did this on a machine that I rarely use.
I am trying to update Command line tools on my mac osx.
But when I run the update command, I get this error:
This doesn’t work either:
What is the exact string I should specify after sofwareupdate -i?
For future travelers, here’s a version-agnostic approach. First, run softwareupdate --list. This will probably take a couple of minutes. When it’s done, you’ll see a bulleted (with an asterisk) output like this:
Find the bullet that refers to the Xcode command line tools. Copy that entire line (except the asterisk…). In the above case, you would copy: Command Line Tools (macOS High Sierra version 10.13) for Xcode-10.1
Then, run the install command (as shown by Brendan Shanks) with what you copied inside quotes:
I’m going to answer a slightly different question here, because this question came up when I searched for a solution to my problem. Hopefully it’ll help someone (and it’ll surely help me next time I run into the same issue).

I wanted to upgrade the command line tools from version 8 to 9. The App Store didn’t suggest this upgrade, and neither did softwareupdate --list.
installed the new version of the tools. But clang --version still gave 8.0.0 as the version number. xcode-select -r and rebooting didn’t solve this issue.
xcode-select -p returned /Applications/Xcode.app/Contents/Developer, and clang --version reported an installation directory under there. I thought I’d start over again.
deleted version 8 of the tools. But xcode-select --install said the command line tools were already installed.
Now, sudo xcode-select -p returns /Library/Developer/CommandLineTools/.
It seems that the problem was that the new version of the tools are installed to a different directory, and xcode-select -r is not clever enough to find the latest version.
Run softwareupdate -i 'Command Line Tools (macOS El Capitan version 10.11) for Xcode-8.2'. The quotes are important.

I faced similar problem on MacOS Mojave version 10.14.3 with Xcode 10.3 installed.
The real problem was, when I installed the Xcode 10.3, I deleted the “Xcode-beta.app” first and then installed the new version. Therefore, when I tried installing CLion for C++ development and configuring it, CMake gave me error And Updating Command Line Tool didnt work for me:
and showed me this response in terminal
Then I tried to check the version of Clang using:
And the response lead me to the real problem i.e. Active Developer path was still pointing to Old version of Xcode that I had already deleted.
Therefore, I switched the active developer path to latest Xcode App installed using:

And everything worked like a charm automatically.

I ran the same command with sudo and that did the trick.
sudo softwareupdate -i 'Command Line Tools (macOS El Capitan version 10.11) for Xcode-8.2'
when upgrading to MacOS Catalina, Version 10.15.* you can install the command line tools for xcode 11.3 like this: