Menu: Editor -> Edit All In Scope (also shows key binding)
Keyboard Shortcut: Control–Command–E
If you want you can change this key binding on Preferences -> Key Bindings -> search for “Edit all in scope”.
EDIT: In Xcode 9, you may want to instead use the fabulous new “Rename…” functionality, which works across classes, also renames files and comments, and gives you a neat preview. As of beta 2, this does not have a default keyboard shortcut, but you can easily give it one.