how to replace word in file in linux code example

Example 1: linux replace string in all files

sed -i 's/old-text/new-text/g' input.txt

Example 2: sed replace in file

sed -i 's/foo/bar/g' hello.txt

Example 3: replace word in file linux command

sed -i 's_oldword_replaceword_gI' document_name.txt