To get the newest root on a mac: this is what Carl and I went through today 

Note that root was installed on my mac by IT earlier

First we tried:

brew update

That gave:

==> Migrating root6 to root

==> Unlinking root6

Error: Error occurred while migrating.

Permission denied @ unlink_internal - /usr/local/etc/root/HistFactorySchema.dtd

Backing up...

Error: Could not link:

/usr/local/share/zsh/site-functions/_brew

Please delete these paths and run `brew update`.

Error: Could not link:

/usr/local/share/doc/homebrew

Please delete these paths and run `brew update`.

Then we tried:

brew upgrade

this actually did work to download some things so that was good but did not really solve the problem 

Then: 

brew install root6

Same problem: 

==> Migrating root6 to root

==> Unlinking root6

Error: Error occurred while migrating.

Permission denied @ unlink_internal - /usr/local/etc/root/HistFactorySchema.dtd

What is a symlink? 

symlink is a file pointer 

points to another location

So, if you had a root on your computer (installed by IT in my case) you might have stuff on there that is conflicting with the new stuff

for me, brew suggested I do this: 

brew link --overwrite root

this gave lots of errors as I did not have permissions to write to lots of directories (as set up by IT) 

so then did

sudo chmod 775 /usr/local/etc/root/*

sudo chmod 775 /usr/local/etc/root/*/*

sudo chmod 775 /usr/local/etc/root/*/*/*

sudo chmod 775 /usr/local/etc/root/*/*/*/*

And then: 

brew link --overwrite root

Hopefully then you get 

Linking /usr/local/Cellar/root/6.10.08... 9717 symlinks created

After this, I had the latest root and it worked