Logic - they just learn by practicing with talking with an accent.

Some of them go to a voice coach, some go and spend a few months in the location where their new accent is used and talk with the locals, and absorb it over time, and some just seem to turn up with a bad comedy attempt and think that will do, like Ross did in Friends for example....

Yes and sometimes they drop the natural ingredients of the voice they have.

This is always fun! I use to be in theater a few years back and found myself having to do a few different accents. I don't know how the real actors/actoresses do it but what I did was once I was assigned the role and what I would watch movies or skits with that same accent. I would sit and practice over and over again with the accent. Takes a while to get it down pat but it's fairly simple though! People might think you're nuts for talking to yourself in a different accent, but it's all in fun!

Movie producers and directors hire what is called a dialect coach. Look for that title in the credits next time you view a movie with some accents in it. When I was a very young man I met and talked with an actor/dialect coach named Robert Easton. My mother had an antique store in the Pasadena area of Los Angeles County(just a stones throw from Hollywood). One day this stately gentleman walked in and a friend of my mom recognized him. He had actually come to look through and purchase some of the old books we had for sale. In talking with him I asked why he had decided to purchase some of the books. He carefully explained too me that in some of these books the author had written dialogue between the characters using an accent. For instance when reading the purposfully mispelled words out loud it sounded like you might be from where the story was written about. He said that this helped him to teach actors how to soubd like they might be from anywhere in the world. VERY INTERESTING AND NICE GENTLEMAN. Sorry for such a long answer but it amazes me how when you read a certain question how a memory that you had almost completely forgotten about pops into your head.

The same way you learn other languages. You have to practice speaking with that accent.

We LISTEN to someone from the area, with the accent we need to learn...THEN...try to relax and speak the same way....Coaches of course often come into play as well...but ultimately...you LISTEN to LEARN.