Normally, when the Android developers release an update for the operating system, you have to wait for your cellphone service provider to release that update for your phone over their network. However, you can use third-party programs to put your phone into download mode and flash the updated firmware yourself. Once your phone is in download mode, you need to get out of it before you can use the phone again.
Finish flashing the new firmware. The process can take awhile, so wait for the phone to reboot itself.
Hold the "Power" button down until the phone shuts down, then press the button again to restart the phone.
Pull the battery out of the phone, then reinsert it. Only do this as a last resort if the phone gets stuck.