Change owner
sudo chown -R OwnerName /path
sudo chmod -R g+rw /var/www/html/
Change Group permission
sudo chgrp -R groupname /path/of/directory
Change owner and group
#sudo chown -R owner:group /var/www/html/
sudo chown -R www-data:www-data /var/www/html/
To add existing user kim to group www-data
sudo usermod -aG www-data kim
id kim
groups kim
Forget -a flag and you will belong to www-data group only
Add a new user to www-data group
sudo useradd -g www-data kim
Add existing user to www-data group
sudo adduser kim www-data