I know this goes without saying, but I will say it.
If you can't su to root, you will have to reboot to single user mode and then execute " passwd root " to change the password. I know that my company has a policy about changing the password/door locks whenever an SA leaves the company.
Also, I think by how you framed the question, that I am assuming that your system is logged in at the console.
I try to never leave the console logged in. just to be safe. I would advise, if that is possible, don't leave root logged in, unless you system is physically secure.
Our company requires root and admin passwd changes every 30 days. It is a real pita! But yes, "passwd root" works. If you are running NIS+ there are a couple other steps to fix the secure RPC passwd (chgkey -p ; nisping org_dir ; keylogin -r).