In-place edit

Modify file in-place (no backup); on macOS add empty string: -i ''

sed -i 's/old/new/g' config.cfg

More sed commands

In-place edit with backup

sed -i.bak 's/old/new/g' file.txt

Edit in-place and save original as file.txt.bak
Delete matching lines

sed '/^#/d' config.txt

Delete all comment lines starting with #
Delete blank lines

sed '/^[[:space:]]*$/d' file.txt

Remove all empty or whitespace-only lines
Print specific line numbers

sed -n '10,20p' file.txt

Print lines 10 through 20 only (suppress default output with -n)
Print lines matching pattern

sed -n '/ERROR/p' app.log

Print only lines that match ERROR