Plenty of theatre schools/actors studios do classes in this. It basically involves focusing in on one sound at a time, with a lot of listening and repetition. If there are no theatre schools near you or they are still Covid-closed, you could look into online classes - but ideally pick one with a live tutor who will listen and correct you.
In the theatre the accent is known as "General American", not being easily placed in any specific region.
https://www.city-academy.com/online/online-american-accent-t...
https://www.thevoicecafe.net/learn-General-American-accent-o...