sed 's/string/string2/' file.txt >file2.txt
This command will set sed in a substitute mode (s), and it will substitute string with string2 in every line in file.txt and save the output in file2.txt
If string is present two or more times in the line it will be substituted only in the firsts occurense and then the sed command will move to the next line. So in this case write g to specify to sed to do the substitution globally.
sed 's/string/string2/g' file.txt >file2.txt
To substitute a character or string with a new line separste the command in two lines:
n/g' file.txt > ile2.txt
This command will substitute every "," with a new line.
If you want to give multiple substitute commands in one go use -e
sed -e 's/string1/string2/g' -e 's/string3/string4/g' file.txt >file2.txt