I need install nightly channel from specific version (1.38.0). How I can do this?
rustup install nightly-2019-10-25
where the date given is the nightly you want.
Note that the rustup date is usually one day later than what
rustc -V reports. The nightly deployment runs just after 00:00 UTC, capturing the latest merged CI build (from the day before).
Rust installed this method returns 1.39.0 version
According to the Rust release notes, Rust 1.38 was released on 2019-09-26, so on the day before that date Rust 1.38 was on the beta channel.
If you want the nightly build which eventually became Rust 1.38, you need to go further back in time to the day before the Rust 1.37 release (which is the day where Rust 1.38 entered the beta channel), and that is 2019-08-14.