how to use sed output to overwrite existin file

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

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

# Finds a specific string with regex and appends content
# overiding existing file. 
sed '/regex/a' <file> | tee <file>
# Example
sed '/\[task_concurrency\]/a 'execute=True'' carbon.cfg | tee carbon.cfg