To change the default currency for a specific client in WHMCS, follow these steps:

1. Log into the WHMCS Admin Area.
2. Navigate to Clients > View/Search Clients.
3. Search for and select the client you want to modify.
4. On the client’s profile page, click on the Edit button.
5. Under the Currency section, select the desired currency from the dropdown list.
6. Click Save Changes to apply the new currency for the client.

Now, the client will be billed in the new currency.