sed -i 's/original/new/g' filename.txt
This basically replaces every occurrence of the “original” string with the “new” string in the file named “filename.txt”
e.g.
sed -i 's/192.168.0.1/192.168.1.1/g' /etc/fstab
Will replace all occurrences of the IP “192.168.0.1” with the IP “192.168.1.1” in the “/etc/fstab” file.
That’s it, Enjoy!
Sources:
Just Some Techie Notes! 
Hey SoCRaT.
Just an additional tip on a side note escpecially when if you are working with lots of linux configs.
If the text that you are trying to replace contains special characters, you can use backslash (\) to escape. Here is an example:
-Lets say you have a file called “test.txt” with text “abc/e123”
# cat test.txt
abc/e123
Now if you want to replace string “abc/e” with “abcde”, you can use the same command with a backslash before the special character (in this case the forward slash):
# sed -i ‘s/abc\/e/abcde/g’ test.txt
Now, the text would be replaced along with the special character will be parsed and replaced.
# cat test.txt
abcde123
Hope this helps! Have a great day!
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