If you have all the slots already populated, then you have to remove some of the modules and replace them.  But if you have an empty socket, you don’t really need to remove any existing module.  Keep in mind, though, that to get optimum performance, modules must be installed in descending order, with the highest capacity module going into the first slot.  Thus, if you bought a memory with a higher capacity than your old one, you need to remove it and populate the sockets accordingly.

There are also some systems that require two matching pairs or in groups of four or more at a time.  So before you buy additional memory, check the specs of your system and inspect what is already installed in your computer.  eFetcher offers upgrade options for a wide variety of systems thru our Memory Configurator.