I've seen several post questions about this operation to change memory or HD so here's my take on taking apart a mac mini.

The correct tool, believe it or not, is a small paint scraper, sharpened. No kidding. You have to pry it open. try to find one that is like 1.5" wide blade, very flexible, and thin.

The process takes some practice and there are more than one ways to do it, but you basically dig in between the metal and white plastic and pry. Once you get one side lifted up about 1/4", insert something to hold it there, remove the blade, and pry from the other side. It will be a lot harder now that the other side is under tension. It will eventually release with a frightening pop.

Carefully work around the perimiter to free the top case. There is nothing attached to the top case to worry about.

When reassembling, pay attention to the kensington lock port on the back. The metal tab needs to be seated in place before you get too close to closing the case, because it will not fall into the correct place if it is the last part of the case to close.

As others here in the forums have found out the hard way, be sure you get all the little cables reattached before you reassemble. And if you have to release the ribbon cable be very gentle with the bail lock that holds it - you really, really do not want to break that.

Before you snap it shut, plug it in and test it. Check the power button, sleep/power light, internal speaker, external headphone jack, and make sure the fans aren't going wild, before you button it up. You won't be able to insert a CD until the case is closed back up fully, test with a CD after reassembly to make sure the case is on fully. Remember to get the lock tab settled in place before snapping it shut.

What you are replacing is not considerd by apple to be a "user replaceable part" and they will have little pity on you if you break something in your attempt. If you are not confident of your skill it may be worth the $30-40 to have a professional do this for you.