Memory modules plug directly into the motherboard so they are always located inside the computer case. Before you can reseat memory, you must power down the computer and open the case so you can access the modules.
Most computers come in either tower-sized models or desktop-sized models. Tower cases usually have screws that secure removable panels on either side of the case but will sometimes feature release buttons instead of screws. Desktop cases usually feature easy release buttons that allow you to open the case but some will feature screws similar to tower cases.
For detailed steps on opening your computer's case, see How to Open a Standard Screw Secured Computer Case. For screwless cases, look for buttons or levers on the sides or rear of the computer that are used to release the case. If you're still having difficulties, please reference your computer or case manual to determine how to open the case.
Note: These steps show how to reseat any kind of desktop memory. There are many different kinds of memory that a PC might use but the reseating process is identical for all of them.