After googling for a while I found a solution. The problem was resolved just by running a terminal command: sudo do-release-upgrade P.S. Now after rebooting everything seems to be working well.:emotion-2:
3-4 months ago I bought a Dell Inspiron N5040 notebook with Ubuntu 10.10 pre-installed. In general I'm very happy with it since everything works OK and I don't feel sorry for migrating from windows to Linux OS. But the only issue I came across lately is that Ubuntu 10.10 is coming to EOL on 10-12 april and I can't see available Ubuntu releases in Updates Manager for upgrading my system.  From reading "About Ubuntu" I got an idea that there should be an option of upgrading ubuntu from within Update Manager but in my case I can't see any available Ubuntu releases in Updates Manager. I wish anyone could share some ideas regarding this matter... would be highly appreciated:emotion-21:
Ubuntu upgrade