upgrade ubuntu 12.04 to ubuntu 12.10 (lower graphics mode)

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http://askubuntu.com – I just upgraded my 12.04 based Dell Inspiron laptop to 12.10 but after the reboot I got a message saying system is running in lower graphics mode. But I'm still not able to access Unity and GNOME. This is with an AMD Radeon HD 7570M graphics card. How do I fix this issue? (HowTos)