Are any of the cartridges empty or not? Always replace the cartridges with the printer on.
If you do not have any low ink alerts, you need to enter the maintenance mode. Press the Maintenance button (has both an ink drop and a paper symbol) and the Power button at the same time. Then open the black printer cover (lift up).
If any cartridges are low or out, just lift up the printer cover. Then press the maintenance button. The cartridge carrier will move the empty cartridge to the replacement position. (If both cartridges are out or you are using the Maintenance mode, the cartridge carrier will move the black cartridge to the replacement position.) Lift up the cartridge clamp (at the back of the carrier for current cartridge to replace; it will rotate towards you.). Then lift the cartridge up; dispose of the empty cartridge. Unwrap your new cartridge and remove the yellow tape (do not touch the IC chip or remove the other tape from the cartridge). Push the new cartridge straight down into the carrier.
Close the cartridge clamp and press the maintenance button to move the color cartridge to the replacement position. Repeat the cartridge replacement steps above for that cartridge. Close the printer cover. Then press the Maintenance button again and wait while the printer charges the ink system.
For pictures to go with the steps, see http://support2.epson.net/manuals/english/ij/stylusc43/REF_G/INK_1.HTM
I hope this helps.