When you create a new client, you’re given the option to enter all these details then, but in effect, this just creates a contact for that client.
If you view your client (e.g. by going to Sales >> View clients >> Select the client), you will see ‘Contact Details’ on the left hand side.
Next to each contact is a magnify glass. If you click this, it will give you the option to update their email address, name and telephone number.