How to decode user password from wordpress mysql database

I just bought 2 wordpress sites with some posts in the databases. I want to use them but i don't know how to log in into the control panel. The guy who sell them to me, say "I don't know any passwords, i forgot it, because this sites are being not in use for long time".
I found 2 useames with admin privilegies in the wp_users table, but the passwords are encrypted. Can i decrypt them, or do something else to use databases ?