Two ways;
First way:
When the grub splash screen is displayed press any key
Select the Linux boot choice and press the 'e' key
Select the kernel line and press the 'e' key again
At the end of the line add a space followed by the number three (3)
When done press the 'Enter' key followed by the 'b' key
Log on as root
Second way:
From the GUI log on screen press the three key combo of Ctrl+Alt+F1
log on as root
type: init 3
Note you may have to press the 'Enter' key after the last green [OK] is displayed.
For yum info read:
http://docs.fedoraproject.org/yum/