[CentOS] restarting mysql databases

I am doing an install of a mysql system, and decided that I needed to 
change some things like host name.  So I have to go back to square one 
with the mysql setup, but can't find any instructions for this.  I TRIED:

mysql_install_db

And it did SOMETHING, but

mysqladmin -u root password 'mypass'

failed with:

mysqladmin: connect to server at 'localhost' failed
error: 'Access denied for user 'root'@'localhost' (using password: NO)'

I tried 'yum reinstall mysql' and that did not help.