The print head and the carriage it rides in looks something like this:
When you get ready to replace the print head, you first need to remove the existing cartridges, then raise the grey lever on the right side. Then you can grab the print head by one of the black handles on either side and lift it out and up.
You can then put the new print head in, lower the grey lever, and install the cartridges.
One word of warning: never leave the grey lever up while the printer is turned on because after a few minutes, the printer will return the carriage to the park position and you don't want the lever jamming into the top of the cover. Either unplug the printer while you're working on it or put the lever back down and close the cover so that it will reset the timer.