xmlstarlet update an attribute
<?xml version="1.0" encoding="UTF-8"?>
<channel name="xfce4-keyboard-shortcuts" version="1.0">
  <property name="commands" type="empty">
    <property name="default" type="empty">
      <property name="<Alt>F2" type="empty"/>
      <property name="<Control><Alt>Delete" type="empty"/>
      <property name="XF86Display" type="string" value="xrandr --output LVDS --auto --output VGA-0 --mode 1680x1050_60.00 --right-of LVDS"/>
    </property>
    <property name="custom" type="empty">
      <property name="<Alt>F2" type="string" value="xfrun4"/>
      <property name="<Control><Alt>Delete" type="string" value="xflock4"/>
      <property name="XF86Display" type="string" value="xrandr --output LVDS --auto --output VGA-0 --mode 1680x1050_60.00 --right-of LVDS"/>
      <property name="override" type="bool" value="true"/>
    </property>
  </property>
</channel>

xmlstarlet edit --update "/xml/channel[@name=xfce4-keyboard-shortcuts]/property[@name=commands]/property[@name=custom]/property[@name=XF86Display]/@value" --value "test" xfce4-keyboard-shortcuts.xml