How to update just one library from the Cartfile with Carthage?

From 0.12 version build, checkout, and update take an optional space separated list of dependencies

For a Cartfile like the following

github "Alamofire/Alamofire"
github "ReactiveX/RxSwift"

You could choose to update one dependency

carthage update Alamofire

or multiple dependencies

carthage update Alamofire RxSwift

If you need to add flags, add them last:

carthage update Alamofire --platform iOS