changing password in main login.

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http://www.ubuntux.org – Ok, i've been reading how to change passwords and such.. but I just need a simple answer.. I have 1 login to sign on upon start up... I NEED to get that password changed. I dont want to make a new login for the other person who uses my computer, its just my boyfriend... he has his own computer. but knows my password. and I just dont want him on it when i'm not home...after what I found today in the history :) so simple answer to a simple question.. how do I just change the initial login password for ubuntu... (Distributions)