Synaptic will not work when started from the xfce menu: the path is wrong...
You can start synaptic either
- as root (even from the menu, or in the shell) or
- gksu synaptic (as an ordinary user with sudo group-membership)

I would do it the old way:
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade

Important: set up the repos first!
BTW: there are ASCII-Beta isos!
There is good documentation in this forum. Find it. Good luck!