Note: Use your gmail account login details to login

2. Click on All tools and then click on Billing.

3. Click on the payment settings

4. Click on add payment method.

5. Add the payment method of your choice

Method 2:-Click on this link https://business.facebook.com/ads/manager/account_settings/account_billing/?act=229233451108511&pid=p1&business_id=472452149929507&page=account_settings&tab=account_billing_settings

You will reach step 4 directly and then follow the steps as mentioned above.