First make sure that the printer is on and the front cover and output tray are open. Next open the paper output cover by rotating it down towards the output tray. The ink cartridge carrier should move to the ink replacement position on the right side of the printer. Push the cartridge lock lever down until it clicks to unlock the cartridge that you need to replace. (The color cartridge is on the left.) Pull the cartridge towards yourself to remove it. Now open the replacement cartridge packaging and remove the protective tape from the cartridge. Push the new cartridge fully into the open spot from removing the old cartridge. (The label should be up and the copper covered sections should be down and facing forward.) Now push the cartridge lever lock up to lock the cartridge in place. If the lever doesn't move, check that the cartridge is fully forward in the slot. Finally close the paper output cover. Wait for the printer to complete the print head clean cycle before using the all-in-one.
Note: only remove and replace one cartridge at a time. Work quickly to minimize the chance of the ink system drying out after removing the protective tape.
Canon has the steps to change any ink cartridge on the Pixma MG4250 support site in the FAQ labeled Replacing an ink cartridge. The full text and pictures are available here: http://www.canon.co.uk/support/consumer_products/products/fax__multifunctionals/inkjet/pixma_mg_series/pixma_mg4250.aspx?type=faq&search=1&searchurl=/support/consumer_products/content/faq/?itemid=tcm:14-1276595
. You can also get the User's Guide and other documentation here: http://www.canon.co.uk/support/consumer_products/products/fax__multifunctionals/inkjet/pixma_mg_series/pixma_mg4250.aspx?type=manuals&language=
I hope this helps.