Step::1 Login to your WHMCS.
Step::2 Hover over the Setup section and go to Products/Services section and click on the Products/Services option.
Step::3 Now click on the Edit icon near the Product group.
Step::4 On the next page jump to the Pricing tab and click on Recurring option.
Step::5 Fill out the details as per your requirement.
Step::6 Click on Save changes.
That's It !Check the video tutorial on the same. Read the blog as well.