i bought a mirro 12Qt. pressure canner and there is no manual. can anyone please tell me know to change the psi?? it says it has 5, 10, and 15. help!

I have the same pressure cooker. Most everything is done on 15 for 15 minutes.
Place the lower plate in the bottom of the cooker so it is raised off the bottom. Put enough water in it to just cover the plate. Place your jars in, cover and turn cover so it locks. Turn on the heat and put the guage on it at the 15 hole. Let it go until the guage "rocks" for 15 minutes then shut it off and allow to cool. After it stops venting, remove the guage with a towel and allow remaining steam to escape. Then you can open it and remove the very hot jars.
Whatever you do... DO NOT OPEN IT WHILE THERE IS STEAM STILL ESCAPING!!!! It can "blow up" on you

thanks

you can download a manual from Presto for $4.99

http://www.cookingandcanning.net/mirrocanner.html

If you have canning questions, I recommend this site- http://www.mrssurvival.com/forums/ubbthreads.php?ubb=postlist&Board=30&page=1
There are women there who work in the extension office in the canning section, and others who have been canning for decades. Any question you have will be answered, and the site owner has been known to stay on the site 'live' to walk someone through their first canning session!