to internal RC oscillator of 8 Mhz.
The internal oscillator is about 1MHz. Once you change to it you won't be able to use serial to down load your code as the baud rates will be screwed up.
To change it you need to change a bit in the fuse settings. See the data sheet for the ATmeg processor you have. You can only do this with a programmer of some description.
Do you still want to do it? If so why?
See this thread:-
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1163418637