Instructions here: http://wiki.maemo.org/Updating_the_tablet_firmware

Also, do yourself a favor and install the tear web browser. Once she's got the firmware updated and is using tear, she won't want to go back to her centro.

And if you don't mind my going off topic, can I ask how she likes the centro. I'm just now considering getting one as my phone/pda, but I want to make sure that the battery life is OK and that there aren't any other goofy bugs. Thanks!