Let's say you have two or more identical strings of data. In this example, the text "Hello" is found more than once.

Hello John Hello Amy Hello Sal

This sed statement will replace only the first occurrence of "Hello" with "Goodbye".

~]# echo "Hello John Hello Amy Hello Sal" | sed "s|Hello|Goodbye|"
Goodbye John Hello Amy Hello Sal

The g (global) flag can be used to replace every occurrence of "Hello" with "Goodbye".

~]# echo "Hello John Hello Amy Hello Sal" | sed "s|Hello|Goodbye|g"
Goodbye John Goodbye Amy Goodbye Sal

The following can be used to only replace the last occurrence of "Hello" with "Goodbye".

~]# echo "Hello John Hello Amy Hello Sal" | sed "s|\(.*\)Hello|\1Goodbye|"
Hello John Hello Amy Goodbye Sal