how to change "§" (section symbol) key to write "`" (back quote)

Solution 1:

You can use Karabiner to remap keys on your keyboard.

<?xml version="1.0"?>
<root>
  <item>
    <name>Section to Back quote</name>
    <identifier>sectiontobackquote</identifier>
    <autogen>
      __KeyToKey__
      KeyCode::UK_SECTION | ModifierFlag::NONE,
      KeyCode::BACKQUOTE
    </autogen>
  </item>
</root>

Solution 2:

An easier way with Karabiner is to just add a new Simple Modification (the first tab in Karabiner):

From key: non_us_backslash
To key: grave_accent_and_tilde (`)

Here's a screenshot of that:

Why is that called non_us_backslash and not §?

The "non us backslash" is probably any key directly below the esc key (or the touchbar) (source: this issue).

On the English International, UK English, Dutch, Polish etc. keyboards, that's the key that produces the section symbol §.