In general your clients will seldom need to use the password. Any invoice you issue on Quick File will contain a link that automatically logs the client in, unless you have the following checkbox marked in the client settings area (this will force that particular client to always log in manually).
If you need to change the login email or password, you can do this from the client detail screen, just click to edit the contact, change the email or password and check the box to email the new details to the client. More details can be found on this here: