This is a very easy step. Basically, you are just taking the cover off of your new case. If you have a plain-jane case, you take a screwdriver and remove the six screws on the back of the case that lie on the edge of the case. Save the screws for later and put them in a place where they will not be scattered and can be easily found. After they are removed, the entire case cover comes off in one piece. With this design, the front of the case does not move. Only the top and sides come off as a cover. In some cases, you may yank the front cover off and then unscrew the case cover from the front. 

If you have a newer case, and possibly a more expensive case, it may come apart differently. The manufacturers of cases have made efforts to make cases "screwless". With this design, you usually take hold of the bottom of the front bezel of the case and give it a nice solid yank. The front then pulls off. It is my experience that this usually requires a few tries and some muscle. The case is pretty durable. Then, the sides lift and slide off. And you just lift the top off. Your case, then, comes off in four pieces. Other such cases come apart the same way, but after you take the front off, the top and sides come off together. 

Each case is a little different in how it comes apart. There are almost as many designs as there are companies that make them. You may find some where you don't even have to remove the front, and rather you just slide the sides off. On others, still, you can remove the whole motherboard mounting plate and card rack combo from the case by sliding it out the back. This is convenient for making quick changes to the system, although you still have to disconnect the various cables to get it out all the way. 

Now that this is done, you are ready to move on.