Replace one substring for another string in shell script

#!/bin/bash
firstString="I love Suzi and Marry"
secondString="Sara"
echo "${firstString/Suzi/$secondString}"    
# prints 'I love Sara and Marry'

EXAMPLE 2:
URL="http://hostname/project/branches/Old_Branch/package"
SRC="Old_Branch"
REP="New_Branch"

echo "${URL/$SRC/$REP}"

from : https://stackoverflow.com/questions/13210880/replace-one-substring-for-another-string-in-shell-script