A papersheet is stuck into the printer, we cannot get it out. How must we open the printer (from behind?)? I have a Canon MX510.
You can use the link below to check for paper jam. The link will be for a different device, but it works exactly as it's described on the page. These printers are exactly the same. Follow the instructions carefully and the paper should be out in no time.
Instructions: http://ugp01.c-ij.com/ij/webmanual/ErrorCode/MX450%20series/EN/ERR/ERR_Other0100.html