I have upgraded my system to Mac OS X 10.6.8 and it says they are no other upgrade available. How can I upgrade to Mac OS X 10.7 from there ? One of my sofware requires it...

Open the Mac App Store and try purchasing Mountain Lion 10.8. If you get told it's incompatible, choose About this Mac from the Apple menu, check if the computer has at least a Core 2 Duo(not Core Duo) or better CPU with at least 2GB of RAM, and if it does, phone the online Apple Store and order a download code for Lion 10.7.

(83620)

you do not use software update. you go to apple web page and do a software update for a cost, from there.

http://www.apple.com/osx/how-to-upgrade/