Give Username Permission to Modify Files on AWS Instance Using chown Linux Command

Using PuTTY, I tried giving one of my usernames access to an instance on AWS using the following Linux command:

chown -R userName /var/www

However, I got the following error message on each folder/file:

chown: changing ownership of ‘/var/www/’: Operation not permitted

How can I give userName permission to modify /var/www? I guess first I need to give myself the power to grant ownership to another username, how would I do that?