If the Business Central connection details are modified on the "Payment Gateway Setup" Page, a notification will pop up on the top of the screen asking if you want to resend the updated details.

You can either use the Yes link in the notification to complete this or you can click on the Send Connection Details Button located under the Actions Menu. 

Using either option, Business Central will check to ensure the Tenant Code and Access Key are populated and then send them to the Web Service URL