I have the same problem. It's actually not related to edit self check in guide. Yes, you can add by text check in method including a description. I think Skip is refering to something else. And I have the same problem.
A while ago, Airbnb introduced a check-in guide with step by step check in instructions with photos, so you can create a PDF for your guests how to check in, which I think that's a great. You can just email the guests the link, and it's easily for them to follow the instructions.
But once created, they failed to add a feature so that you can update the information when you changed the keypad code for example. There is no way to update that info.