Select the target, goto Build Settings > Swift Language Version:

All the above answers are answering the wrong question. When using Xcode 9, you are using the Swift 4 compiler (even if you are using it to compile Swift 3 code) to fix this, recompile using the appropriate compiler/Xcode.

If you are trying to do this via commandline, you can use sudo xcode-select -switch to switch between xcode versions. Good luck!

Update for Xcode 10.1 and Swift 4.2

Swift 3, 4, and 4.2 targets can coexist and link together.

You decide when and if you’d like to migrate on a per-target basiswhen it makes sense for your project. While migrating to Swift 4.2 isdefinitely encouraged, it’s not an all-or-nothing process, as Swift 3,4, and 4.2 targets can coexist and link together.

Different cocoapod and different project version can also coexist.

You wish to have different cocoapod version as compared to your project version then you should make following changes at the end of your pod file:

post_install do |installer|    print "Setting the default SWIFT_VERSION to 4.2\n"    installer.pods_project.build_configurations.each do |config|        config.build_settings['SWIFT_VERSION'] = '4.2'    end    installer.pods_project.targets.each do |target|        if ['SomeTarget-iOS', 'SomeTarget-watchOS'].include? "#{target}"            print "Setting #{target}'s SWIFT_VERSION to 3.0\n"            target.build_configurations.each do |config|                config.build_settings['SWIFT_VERSION'] = '3.0'            end        else            print "Setting #{target}'s SWIFT_VERSION to Undefined (Xcode will automatically resolve)\n"            target.build_configurations.each do |config|                config.build_settings.delete('SWIFT_VERSION')            end        end    endend

Sometimes even after you run pod update your xcode forcefully update your cocoapod's project version, then in that case select cocoapod pod project and change the swift version in the build settings.