Update zsh on macOS Mojave
Not exactly. You can't (although that can be overridden) and should not update anything in /bin as that is controlled and updated by Apple.

However you can install zsh from elsewhere and put it in another place and use that path to start it from Terminal.

One way is to download the source from here and compile it.

However it is probably easier to install from a package manager like Macports or Homebrew both of which  will have compiled up the binary.