This is step by step about how to reset Ubuntu password to a blank one. Please use this wisely.

1. Boot from Ubuntu live CD/USB
2. Go to terminal, type : gksu nautilus
3. Navigate to root partition of installed Ubuntu on hard drive
4. This will be the most important part
   
   • Open file /etc/shadow with text editor
   • Search row started with user name that wanna reset. Change weird phrase between first and second colon with 'U6aMy0wojraho'. For example I wanna reset ifnotnull password, so I change this row :
     ifnotnull:$6$PrT8E$2Ti.V4/gG442ukKxc.abR5VXm1:15428:0:99999:7:::
     to :
     ifnotnull:U6aMy0wojraho:15428:0:99999:7:::
5. If it done well, it will change password to a BLANK one or no password there.
6. Now reboot into installed Ubuntu and login with a BLANK password

*Credit : psychocats.net, ubuntuforums.org, ubuntubuzz.com