Customised menu that changes depending on Child Page

Hi,
I’m new to the forum, and “slightly” new to CMS editing and WordPress (I have experience making simple WordPress sites, average in Bitweaver and Textpattern, and starting on Joomla as well.)

I have a question.

It’s regarding this website, (test URL: https://talystinsite.com)

If you will please navigate to “About Talyst” on the top Menu, you will notice the page has a side navigation which contains the sub pages of “About Talyst” main-page.

My problem is, if I try to switch to another page, for example, Resources page, the same menu from about Talyst appears… not the Child pages of Resource page.

What PHP codes will enable me to customise the menu so that each parent page, the side menu that appear will be it’s corresponding child-page?

Please help, and thank you ??