To edit contact details from your CRM Leads, without leaving the Leads list, you can click on the Client or the Contact name (clickable links are in blue), and it will open up the drawer on the right hand side of the screen.
To edit the Billing details, click on the Edit pen at the top of the drawer.
To edit an individual Contact's details, click on the Contacts tab, and then the Edit pen for the contact you would like to edit.
Once that is done, you can click Save, and click off the drawer and it will close. All of this being done without having to leave the Leads list.