tldoc tlmgr
but basic operation is pretty straightforward. The manager needs to know where to download stuff from; the canonical setup is
tlmgr option repository http://mirror.ctan.org/systems/texlive/tlnet
which passes the decision to the mirror selector. You can (of course) specify a particular archive or mirror that you “trust”, or even a local disc copy that you keep up-to-date (disc space and bandwidth are so cheap nowadays, that a “home mirror” of CTAN is a feasible proposition).
To update a single package, use:tlmgr update <package>
To update everything you already have in your installation, use:
tlmgr update --all
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=inst-texlive