How do I get jammed paper out of the back of it?

This is from the user guide available on the HP.com website:
This section contains the following topics:
• Clear paper jams
• Avoid paper jams
Clear paper jams
Paper jams can occur either inside the printer or in the automatic document feeder.
CAUTION: To prevent possible damage to the printhead, clear paper jams as
soon as possible.
TIP: If you notice poor print quality, clean the printhead. For more information, see
Clean the printhead.
Use these steps to clear a paper jam.
To clear a jam inside the printer
1. Remove all media from the output tray.
CAUTION: Trying to clear a paper jam from the front of the printer can
damage the print mechanism. Always access and clear paper jams through
the automatic two-sided printing accessory (duplexer).
2. Check the duplexer.
a. Push the button on either side of the duplexer, and then remove the unit.
b. Locate any jammed media inside the printer, grasp it with both hands and
then pull it towards you.
c. If the jam is not there, push the latch on top of the duplexer and lower the
cover. If media is jammed inside, carefully remove it. Close the cover.
d. Reinsert the duplexer into the printer.
3. Open the top cover and remove any debris.
4. If you have not found the jam and have Tray 2 installed, pull out the tray and
remove the jammed media, if possible. If not, do the following:
a. Ensure the printer is turned off and disconnect the power cord.
b. Lift the printer off Tray 2.
c. Remove the jammed media from the bottom of the printer or from Tray 2.
d. Reposition the printer on top of Tray 2.
5. Open the ink cartridge access door. If there is paper remaining inside the printer,
ensure the carriage has moved to the right of the printer, free any paper scraps or
wrinkled media, and pull the media towards you through the top of the printer.
WARNING! Do not reach into the printer when it is on and the carriage is
stuck. When you open the ink cartridge access door, the carriage should
return to its position on the right side of the printer. If it does not move to the
right, turn off the printer before you remove any jam.
6. After clearing the jam, close all covers, turn on the printer (if you turned it off) and
resend the print job.

That's about as much as I can find for you. If you look in you user guide which is on a CD that came with the unit, you can go to page 152 through 154 to see diagrams that go with these instructions. Good luck!