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WPCBlaze has it mostly for question 2. Just to make sure we dont confuse you, dont go into your database, go into you web host's server which you should be able to to through, like he says ws_ftp or something similar.
For the first question, unless you want to go round editing your scripts, what i suggest you do is go through your ftp to yoursite.com/themes/default and find logo.gif. Where your logo is, save it as logo.gif and replace this one. Remember that your logo has to be 150x80 pixels. If you still dont understand then send me a pm and i will do it for you.