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.
Are you trying to improve your own pronunciation or the English pronunciation of the Americans? I really hope for the latter, because it would make the world a better sounding place :)
👤 rceDia
Practice, practice, practice. Join a Toastmasters International Club (helps with public speaking). There are TM clubs all over the world.