How to sign out of App Store

Settings><Your Account>Media and Purchases>Sign Out.

Perhaps she can’t, just like I can’t, because the sign out option that was below the list of updates in iPadOS 14 is not there in 17. Where has the sign out button been moved?

Why can't you? What are you trying? Where does it fail to do as expected? Do you see an error message? If you do, we need to know exactly what it says. We really need better information in order to be able to help you.