The first answer is correct. Though having a copy of the owner's manual is always a very good idea. It is also true that most bolt action rifles have a release mechanism to extract the bolt for cleaning and maintenance.
Lower the lever and remove the lever retaining screw, pull the lever straight down out of the firearm. Slide the bolt out of the reciever, being careful not to loose the extractor. Reasemble in reverse. Here is a link to the owner's manual that may help. It is printable. Good luck. http://stevespages.com/pdf/marlin_336_444_1895.pdf
Serial number is stamped on the lower tang of the receiver. It is covered by the lever. Move the lever to see it.
Your serial number indicates that your model 782 rifle was made in the year 1981.
The prefix letter K to your serial number indicate that your Marlin model 336SC (sporting carbine) was made in the year 1953. Without a detailed description of your Marlin rifle's overall condition, it is difficult to assess a value. As a general rule these Marlin rifles will bring between $250 and $400 on today's market depending on overall condition being good or better with a good bore, and wood.
Your Glenfield model 75C will go for between 50-75 dollars on today's market.
Your rifle was made in the year 1996,with the serial number that you have supplied.