give write access to two users

    sudo usermod -a -G www-data <some_user>
Then set the correct permissions on /var/www.

    sudo chgrp -R www-data /var/www
    sudo chmod -R g+w /var/www

Additionally, you should make the directory and all directories below it "set GID", so that all new files and directories created under /var/www are owned by the www-data group.

    sudo find /var/www -type d -exec chmod 2775 {} \;    
Find all files in /var/www and add read and write permission for owner and group:

    sudo find /var/www -type f -exec chmod ug+rw {} \;