How to update a cloned git repository

Hi folks

Probably a beginner's question...

If I did a
    git clone git://guthub.com/git/git.git
and worked on some own branches of pu
    git checkout pu;git checkout -p mybranch;hack;hack;...;git commit -a -s
how to update my repository once the the one on github changed?
A plain
    git pull
or
    git fetch;git merge
keeps failing on my with lots of conflicts, none of which relate to any of 
the changes I did (and hence wouldn't know how to resolve)

Bye, Jojo