Neovim and its installation instructions are available from GitHub at
https://github.com/neovim/neovim. You can either download the binary, or install it through one of the package managers. The installation instructions are rather detailed, and may change rather quickly, so you should give them a read at
https://github.com/neovim/neovim/wiki/Installing-Neovim.
If you are working on Debian based Linux distribution, you can install Neovim by running:
$ sudo apt-get install neovim and
$ Python3 -m pip install neovim to add Python3 to support neovim.
All of the commands ...