When you switch the printer on, there should be a red flashing light with a button next to it on the left hand panel (near the top).
Lift the top to access the print mechanism and cartridge carriage.
Press the ink button once, the carriage will move from the right to the left, it will stop in front of a triangular protrusion which is the indicator. This indicator will be pointing at the cartridge that needs replacing (so make a mental note).
Press the ink button again and the carriage will travel back to the right and settle in a gap which allows you access to the cartridges.
Remove the cartridge indicated by pressing the front clip of the cartridge in and lifting it out. Then remove the yellow sticky tape covering the air vent on the cartridge. If there is a insert placed between the cartridge and cartridge clip, then remove that also. Now insert the replacement cartridge in the ink carriage and gently, but firmly press down on the cartridge until you feel it click into place.
Press the ink button again and the ink carriage will move back and forth, go through a cleaning cycle before settling down on the right hand side out of reach.
(If more than one cartridge needs to be replaced, then each time you press the ink button, the carriage will move to indicate the other cartridges in turn, before moving over to the right hand side to allow you access to the cartridges for replacement)