Is there a command to update Cargo to the latest official release?

You should update rustc and cargo based on how you installed it. If you used rustup, a rustup update should suffice. If you used a package manager or a binary installer, check those sources for an update.

rustc and cargo are shipped together, but that doesn't mean that their versions need to match. In fact, they do not match until Rust 1.26.0, when the Cargo binary was changed to print the Rust version.

I have the same versions of rustc and cargo that you do; those are the ones that correspond to the Rust 1.9 release. There's nothing to worry about.

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If you really want to, you can download a nightly version of Cargo or compile your own. As long as your version exists in your PATH before the older one, it will be used.

I used to do this with my local Rust builds in order to have a version of Cargo at all, although rustup now automatically uses the cargo from the most recent stable version when there isn't one available in the current toolchain, which is nice.

tl;dr command rustup update will update both Rust and Cargo:

$ rustc --version
rustc 1.27.2 (58cc626de 2018-07-18)
$ cargo --version
cargo 1.27.0 (1e95190e5 2018-05-27)

$ rustup update stable
info: syncing channel updates for 'stable-x86_64-unknown-linux-gnu'
info: latest update on 2018-08-02, rust version 1.28.0 (9634041f0 2018-07-30)
info: downloading component 'rustc'
info: downloading component 'rust-std'
info: downloading component 'cargo'
info: downloading component 'rust-docs'
info: removing component 'rustc'
info: removing component 'rust-std'
info: removing component 'cargo'
info: removing component 'rust-docs'
info: installing component 'rustc'
info: installing component 'rust-std'
info: installing component 'cargo'
info: installing component 'rust-docs'

$ rustc --version
rustc 1.28.0 (9634041f0 2018-07-30)
$ cargo --version
cargo 1.28.0 (96a2c7d16 2018-07-13)