Filter areas with respect to city

I am using WP-Rentals theme in which all areas are displaying in areas drop down field.

I want to get areas with respect to city. If I selected Karachi then only Karachi areas should show in area drop down field.

Kindly help me to achieve this as I am facing this problem.

Waiting for your helpful reply.

Thanks

The page I need help with: https://staging.venrooms.com/advanced-search/?property_city=&check_in=&check_out=&guest_no=&wpestate_regular_search_nonce=e7a1e27b3a&_wp_http_referer=%2F&submit=Search