I understand you to mean printing the pages in reverse order (from last to first), because the pages come out face up in the output tray.
This is often done from the applications rather than from the printer; for example, Adobe Reader and OpenOffice both have options from the print dialog to print pages in reverse order.
Alternatively, you can open a command line terminal and use the lpadmin
command to change the defaults for the printer, ie:
sudo lpadmin -p CANON -o outputorder-default=reverse
The sudo will give you adminstration access to run the lpadmin command (and will ask for your password). The name 'CANON' should be replaced with the name of the printer queue you are changing.
To change it back, use:
sudo lpadmin -p CANON -o outputorder-default=forward
Each printer has a Postscript Page Definition file (.ppd) in /etc/cups/ppd which defines a lot of options. The forward order has been made the default even for face-up printers because the printing is faster that way (the print job doesnt have to wait for the last page before it begins printing).