Step::1 Login to your WHMCS.
Step::2 Begin by going to Setup and then Products/Services.
Step::3 Click on Servers.
Step::4 Now double tab on the server you want to set as default server.
Step::5 Now after double tapping on the server you will see a star mark beside the server which denote that the server is the default server.
That's It !Check out the video tutorial on the same. You may read the blog as well.