Hi,

Either you can do it from the front end or by the code. If you are doing from the front end, see the below image,

Navigate to Contents -> Edit Menu -> Select the menu and clicking the edit button you can change the URL.

If you want to do the same from code,  see the original code for the menu contacts,

<record id="menu_contactus" model="website.menu">
    <field name="name">Contact us</field>
    <field name="url">/contactus</field>
    <field name="parent_id" ref="website.main_menu"/>
    <field name="sequence" type="int">60</field>
</record>

here what you have to do is that, in your module, update the code like this,

<record id="website.menu_contactus" model="website.menu">
    <field name="url">/customurl</field>
</record>

Thanks