I don’t think you’ve done anything and I don’t think there’s anything to undo.
There are several reasons why this kind of thing can be happening. Most of them boil down to an improperly constructed email message on the part of the sender. In other words, it’s not you, but the sender.
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From what I can see (and I did go looking when I started researching your question), Outlook.com doesn’t have the ability to switch between plain and HTML format when you receive email. It displays what it considers to be the “best” version if there’s more than one.
And if that’s HTML, then you’re already seeing the HTML version of that email.
The message in the message
I believe that this message to turn on HTML is not from Outlook.com at all, but is actually part of the email. I know that’s kind of confusing, but it was probably placed there by the person who wrote that email.
They may just put it there for everyone. It ensures that users who have email programs that may not display HTML by default change it.
It’s also possible that this is text that’s displayed when images are turned off. It’s unlikely but it’s possible.
I could see someone authoring their email that way. It’s misleading and inaccurate and the email author might just be using that instruction to have you view images. Either way, I don’t think there’s much for you to do.
Links in email
My belief is that there’s no link in the email body because it’s the right thing to do.
I’m always a little concerned when a bank or other financial institution includes a link in email that is sent to me. Why? That’s what spammers do. That’s what phishing attempts are all about. And by having you click links in email, the bank is training you to click links in email.
You shouldn’t do that, specifically when it comes to banks or other financial institutions.
When you get a message like that, you should read it and then on your own, separately, fire up the browser, go to your banking site yourself, and login. That way, you know you’ve gone to your bank and not some misdirection that was included in that original email.
So, I really don’t think there’s anything for you to do other than perhaps change your habits a little bit and go to the banking site directly without relying on a link in email.