This post will explain how to install the Bourne-Again-Shell version 5.1 for Linux (and Unix), independend of the CPU architecture. 
I expect you to have the common build tools already installed. 

This post is an update of Bash 5.0 installation for Linux

wget http://ftp.gnu.org/gnu/bash/bash-5.1.tar.gz

tar xf bash-5.0.tar.gz

cd bash-5.1

./configure
make

sudo make install


and finally, to run sh