A lot of people think that each other and one another can be used interchangeably. But actually, they mean different things and they're used in different ways. The easiest way to remember it, is that each other is used when we are talking about 2 people or 2 things and one another is used when we are talking about more than 2 people or a group of things.
Here's an example of how the 2 of these phrases would function in a sentence. My mom and I talk to each other. Well, my mom and I are just 2 people, so we talk to each other.
However, my family members write to one another. In this case I am talking about my entire family, more than 2 family members and when we correspond, we correspond with one another.
So hope that clears up the difference between each other and one another and helps you choose the right one for you.