The easiest way to edit an existing clients information is to go to Main->Lookup Client:
Then either use the search fields available at the top of the screen to narrow your search results or just hit the search button to generate a list of all your clients:
Once you get your list of results, click on one of the client's names to edit their personal information.
You'll be redirected into their client info screen where you can edit the information on the Client Info or Address Info tabs and manage their appointment history and payment history.
After you've made all the changes you'd like to, hit the "Save" button at the bottom of the client info screen and any changes you made will show up the next time you generate a client list.