Open up App Store
Look in the top right for the updates section (may also be in lefthand column "Updates"..)
Find Xcode & click Update
softwareupdate --list see the list of outdated software.
softwareupdate --list
softwareupdate --install --all update all outdated software.
softwareupdate --install --all
softwareupdate --install <product name> update the software you named.
softwareupdate --install <product name>
Sometime I don't see the Xcode update in Updates tab in AppStoreSo I search Xcode in AppStore
Updates tab
AppStore
Then click update and it update