I found, default elixir compiled with 19. Beacuse of asdf i saw Elixir 1.6.1 (compiled with OTP 20)

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Well, on my system (arch linux) it gives OTP 20, Elixir 1.6.5 (with OTP 20).

So you need to be more specific about which Operating System/Distribution you are talking about.

But in general, the process depends on your distribution or the third party maintainer of the repository.

If you want bleeding edge, either switch to a bleeding edge distro (such as Gentoo or Arch, but even there it sometimes takes a week or two), use an environment manager like asdf or compile from source.

For the last two items the usual considerations about not using the distros package manager apply of course.

Using asdf you can asdf install elixir 1.6.6-otp-21 (or use other otp version suffix)