I have 2 nav menus and can't get rid of one

Hi there. I am new to WP. I have been asked to update an existing WP site which uses the sliding door nav menu. I have sliding installed and working fine. but there is a second nav menu under the sliding door, which I can’t seem to remove. The only menu I have set up in the ‘Menu’s section is the sliding door, so I am not sure where it is getting the data for all the other stuff, under the sliding door. You can see the site here:

https://www.nwmaacademy.com/demo/

Any tips or a good tutorial would be great, thanks.