That's one of the easier things to do:
Lua Code:
local label
label = MyFragmentLabel
-- or --
label = MyFragment:GetNamedChild("Label")
-- or --
label = GetControl(fragment.control, "Label")
label:SetText("Hello World!")
You named your label control "Label" and it is a child of your "MyFragment" control, so you can just access it via its global variable or relative to the control.