The debian repositories that ship with raspbian come with an outdated version of neovim. Your best option is to compile neovim from github

Don't forget to uninstall neovim if you installed it through your package manager previously.

sudo apt-get install -y ninja-build gettext libtool libtool-bin autoconf automake cmake g++ pkg-config unzip curl git wget
git clone https://github.com/neovim/neovim
cd neovim
make CMAKE_BUILD_TYPE=Release
sudo make install