Why would you need to modify an existing Client account? You will want to modify a Client account if you want to change the account's name or password.
Go to the computer running the Backup Server software at your location.
Run Backup for Workgroups and select the Clients panel.
Highlight the name of the Client account that you want to modify.
Press the Modify button.
Make changes as desired and press OK when finished.
Remember, if you make any changes to a Client's login credentials, be sure to pass this information on the the person who uses this account to backup their computer. They will need to visit the Tools>Account Information dialog to enter the same information used in the Backup Server software in order to access their account stored on the Backup Server.