As a 20i reseller, you can buy a discounted WHMCS license. If you have a pre-existing WHMCS license, you'll then need to change the license key in your WHMCS system.
To do this, log in to the admin area and go to Help > Change License Key.
You can then enter your new license key, along with your admin login details to verify the change.
If you are getting a 'license invalid error' - so that you are unable to login - there is a link to the Change License page from that error page.
Further instructions can be found at: WHMCS Support - Licensing.
If you don't want to worry about licenses at all, you can use 20i's free alternative to WHMCS, HostShop!