Blackbaud CRM holds the HMRC Gateway URLs in the AppSettings section of the web.config file. Under this section, you can amend the URLs.

The following should be changed:
Key:     HMRCR68LiveGatewaySubmissionUri
Value:   https://transaction-engine.tax.service.gov.uk

Key:     HMRCR68TestGatewaySubmissionUri
Value:   https://test-transaction-engine.tax.service.gov.uk

The HMRC section of the web.config should look similar to the below:
web.config_hmrc