Step::1 Login to your WHMCS.
Step::2 Hover over the Clients section and click on View/Search Clients option.
Step::3 Now select the Client form the list of clients.
Step::4 On the next page go to Client Profile tab.
Step::5 Now go to Currency option and change the currency from Drop-down.
Step::6 Click over Save Changes.
That's It !
Check out the video tutorial related to the same. You may check out the blog as well.