First, Dell has made 3 kinds of Dell Dimension cases that I know of at this moment. I'm working with case model #2. Here is a picture of what I'm working with...
First, unplug all of the cords leading into the computer and put the computer in a place that is convenient for you to work, such as in the floor, or on a worktable.
When you turn over the case you should see this...
As you can see on this case, which is mine, I have removed all of the PCI slot covers. I did this for better cooling due to an overheating video card. Now let's get down to business. If you have over looked your case, you can see that there is two tabs, one on top and one on bottom, and there is a screw that you can twist out with your fingers.
What you need to do:
(1) - Twist the screw with your fingers until it comes out of the metal part of the case, but not the plastic shell, otherwise you'll go through heck trying to get it back in.
(2) - Push in both tabs on top and bottom and pull towards the back of the case so the side cover comes off. It will not slide off completely, but will move back about an inch, from there just lift it up like a car hood, and it should come off.
Once you take the side cover off, this is what you should see...
Now, you're looking for a black case that covers the processor. It looks like this...
There should be two tabs that connect the cover to the case fan. Push these tabs in, and lift up on the cover to reveal this...
Your case fan will more than likely have a case fan grill on it, and will probably be more dirtier. These pictures were taken after a full system cleaning. Your processor will probably have some dust and junk on it too. You might consider cleaning your case fan and your processor too if this is the first time you've ever opened your case.
Now, look at the processor. You should see two tabs. Push those in until you hear a "Click". Now, gently, but with a little bit of force, pull the processor up and out of it's slot like this...
Now, just put the processor back in the same way you took it out. Don't forget to pull the tabs back out and secure the processor into place, and put the processor case back on.
That's it! You've sucessfully reseated your Slot 1 Pentium 2 or 3 processor. Congratulations!