[sublimetext2] How do I replace text in a selection?

As @JOPLOmacedo stated, ctrl + F is what you need, but if you can't use that shortcut you can check in menu:

  • Find -> Find..

    and there you have it.
    You can also set a custom keybind for Find going in:

  • Preferences -> Key Bindings - User

    As your request for the selection only request, there is a button right next to the search field where you can opt-in for "in selection".