MacOS – How to flip the dash and underscore keys in macOS Sierra

To make the keyboard behave differently, you need a different keyboard layout.

You could create one by editing an XLM file manually, or you could use a third-party tool to create the keyboard layout (you don't need to be running a third-party program indefinitely once they layout is created and installed).

Ukelele is known for making it fairly easy to create new keyboard layouts for the Mac.

In any event, you want to switch the number keys (0-9) and the symbols attached to them. Save your new keyboard in ~/Library/Keyboard Layouts, and then select it as your keyboard.

Install and launch Karabiner. In the menubar open the menulet Karabiner -> Preferences. Then hit the tab "Misc & Uninstall" and "Open private.xml"

Paste the following lines into the opened xml file:

<?xml version="1.0"?>
<root>
    <item>
      <name>Swap Minus and Alt+Shift+Minus</name>
      <identifier>remap.minus_and_optionlshiftlminus</identifier>
      <autogen>
        __KeyToKey__ 
        KeyCode::MINUS, ModifierFlag::NONE,
        KeyCode::MINUS, ModifierFlag::SHIFT_L | ModifierFlag::OPTION_L
      </autogen>
      <autogen>
        __KeyToKey__
        KeyCode::MINUS, ModifierFlag::SHIFT_L | ModifierFlag::OPTION_L,
        KeyCode::MINUS,
      </autogen>
    </item>
</root>

and save the file.

Return to the tab "Change Key", hit "ReloadXML" and enable "Swap Minus and Alt+Shift+Minus". It should be the first entry listed.

If you are left-handed you may replace _L by _R in the xml file.