How do I now change the HMRC credentials (which are wrong) on QB so that I can then tell HMRC to link up with QB for making tax digital?

Hi there, @19ali69-800ks.

You'll want to go to your Account and Settings and change your HMRC credentials from there. I'm more than happy to guide you.

In your QuickBooks Online (QBO) account:

1. Go to the Gear icon.
2. Select Account and Settings.
3. Go to the Payroll section.
4. Click the Edit icon next to Online filing with HMRC.
5. Ensure the Gateway ID and Gateway Password.

Here's an article you can read that can guide you through the detailed steps: Turn on RTI reporting to HMRC in QuickBooks Online Standard Payroll.

I'm also adding this article that provides detailed information on how to authorize QBO to interact with HMRC.

Let me know in the comment section if there's anything I can help you with. I'm always around to help. Have a great day and keep safe.