You can't. One of the dumb things from Vista to 7 was it was a complete re-install.
The best you can do is backup your files [My Documents, My Pictures, etc] and then install 7. You'll still have to re-install software, but you won't lose your personal stuff.
But, I haven't look in a while. Somebody may have created an update tool to assist with -- may want to do a quick Google search before you take my advice
Nick