In case you need to create a new client, you can find information at this article:
http://help.rentlysoft.com/en/articles/5313631-how-to-create-new-clients
To edit existing clients:
Select Customers > All customers
Click on the pencil of the client you need to edit
Modify the data
Remember to always save changes
In case you do not have permission to proceed with this transaction, inform your administrator so he can enable you the "Can Administrate Customers" permission.