The equivalent for ‘www-data’ for CentOs

You don't want them owned by www-data. Apache initially runs as root and then drops privileges to www-data. You don't want your web content writable by the user that owns the apache processes. This creates a security vulnerability.

In the event that Apache is breeched, ideally the www-data user will have no access to any of the system. This is the most secure configuration. If the web content is owned by www-data, if Apache is breeched then the attacker can overwrite any of your web content.

Your web content should be owned by a normal user (this excludes nobody, www-data and root). Only things that need to be writable by Apache should be owned by www-data.

As I searched through the list of unanswered questions for ones that I might be able to answer, I saw this question’s title and I figured that it might be an issue with file attributes. As detailed in the question’s comments, the immutable attribute had been set and @slm posted the solution:

sudo chattr -i /var/www/update

However, @slm also asked a very pertinent question, “why is that bit set in the first place?” The first time I learned of the immutable bit was when I was asked to look at a server that had been rooted. The binaries for ps and lsof had been replaced with modified versions (in order to hide other malicious processes) and the immutable bit had been set to prevent any future OS updates from replacing the malicious executables with updated genuine versions.