@MrsLaurelle1 -- download, install, and run the free SPECCY software.
Expand its "RAM" branch, and expand the section for one of the sticks of RAM.
You should see the current type of RAM (DDR? DDR2? DDR3? DDR4?) and the "speed" of the RAM, and the capacity of the stick, and the number of "empty" RAM sockets.
You need to purchase the same type, and to select RAM that is NOT SLOWER than the current RAM.
Depending on the number of sockets, you could add identical capacity and identical speed RAM into a vacant slot, or if there is only one socket, you will have to replace the current RAM by a larger-capacity stick of RAM.
Note proper electrostatic procedures for a desktop computer:
* shutdown Windows
* disconnect the power cord
* press the ON/OFF button to drain any "residual" electricity
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