Update:

Use the following command is much more effective!

chsh -s /usr/local/bin/bash

Below is the original post:

brew install bash # install the 5.0 version bash with `brew`:

Then, edit the /etc/shells file and add the 5.0 version bash's PATH on the top of the shells:

➜ cat /etc/shells 
# List of acceptable shells for chpass(1).
# Ftpd will not allow users to connect who are not using
# one of these shells.

/usr/local/bin/bash
/bin/bash
/bin/csh
/bin/dash
/bin/ksh
/bin/sh
/bin/tcsh
/bin/zsh