# To replace all occurrences of "day" with "night" and write to stdout:
sed 's/day/night/g' file.txt

# To replace all occurrences of "day" with "night" within file.txt:
sed -i 's/day/night/g' file.txt

# To replace all occurrences of "day" with "night" on stdin:
echo 'It is daytime' | sed 's/day/night/g'