How can I change security settings for 'Client Site'?

You can change the access level to your client site under Client Site Settings. Options available are Login with 'Passcode' and 'Auto Login'. 

Go to Settings > Client Site: 

1. Login with Passcode means that your client will need to use the secure passcode sent to their registrered email address in order to login into Client Site. 
2. Auto Login will remove the passcode requirement and clients can access the website without security verification of their account.