Search the addons folder for the text you want to change and make note of the <record>s the text appears in. Then you can override the help field of all those records in your own module.

For example, the default text in Sales module is defined inside the base module, and it can be overridden with the following XML:

    <record id="base.action_partner_form" model="ir.actions.act_window">
        <field name="help" type="html">
          <p class="oe_view_nocontent_create">
            Some text that goes next to the arrow icon.
          </p><p>
            Some more text
          </p>            
        </field>
    </record>