Rust 2015 has a theme of "stability". It commenced with the release of 1.0, and is the "default edition". The edition system was conceived in late 2017, but Rust 1.0 was released in May of 2015. As such, 2015 is the edition that you get when you don't specify any particular edition, for backwards compatibility reasons.
"Stability" is the theme of Rust 2015 because 1.0 marked a huge change in Rust development. Previous to Rust 1.0, Rust was changing on a daily basis. This made it very difficult to write large software in Rust, and made it difficult to learn. With the release of Rust 1.0 and Rust 2015, we committed to backwards compatibility, ensuring a solid foundation for people to build projects on top of.
Since it's the default edition, there's no way to port your code to Rust 2015; it just is. You'll be transitioning away from 2015, but never really to 2015. As such, there's not much else to say about it!