Question: I don’t know how to replace the printer cartridge nor do I know the number for the cartridge use in Canon MG 3620 printer.
Answer: It’s easy. The Canon MG3620 printer uses one black (MG-240) and one colored (CL-241) cartridge. It comes in standard size and XL size which gives more prints than standard one. You can buy them from here Canon’s PIXMA MG3620 Compatible Ink cartridges page.
Here is canon’s official instructions on how to replace MG-240, CL-241 cartridges in the Canon MG3620 printer.