This worked for me:
$ rustup component remove rustfmt-preview --toolchain=stable-x86_64-pc-windows-msvc
info: removing component 'rustfmt-preview'
warning: during uninstall component rustfmt-preview-x86_64-pc-windows-msvc was not found
$ rustup component add rustfmt-preview --toolchain=stable-x86_64-pc-windows-msvc
info: downloading component 'rustfmt-preview'
info: installing component 'rustfmt-preview'
$ rustup run stable rustfmt --version
rustfmt 0.99.1-stable (da17b689 2018-08-04)