You can't upgrade a program from synaptic higher than intrepid repos.Yes,if you want a latest release you better try here:
http://www.getdeb.net/ ,
http://www.debian.org/distrib/packages ,
or even here some times: http://linux.softpedia.com/ ,
and here: http://linuxappfinder.com/ .
If the new program or app have more dependences you must completely uninstall the old one from synaptic before install the new one.