How To Convert Membership Type of an Existing Contact to Client

Go to Members, then click on 'Member List'

You will be redirected to the Contacts page. Scroll down to 'Contacts' tab and click 'all'

Select the contact that you want to upgrade to a client

You will see that the upgraded contact will be removed from the contacts list. 

It is now transferred on the Clients list and you will see that the member type has been changed from 'Contact' to 'active Client'