You can't. The best way is to back up your home and do a fresh installation.
Then move all your home data back and install any apps that you used to have
installed. The only way to upgrade is from one release to the next. You
cannot skip releases. The only exception to this is going from LTS to LTS,
that would be from 8.04 to 10.04 when Lucid Lynx is released next April. I
do not advise that you upgrade from 8.10 to 9.04 and then to 9.10 because
there is too much potential for something to go wrong.