Which C# API?
Update a hotspot user in what way? What is it you want to change?
The API commands you'll likely need is something like this:
"/ip/hotspot/set"
"=.id=" + userId
"=password=" + newPassword
"=email=" + newEmail
".tag=deantwowashere"
The ".id" is used to specify what user you want to change, I am unsure if you are able to use the name of the user or if it has to be the ID.
But without more information it is hard to help you more. Since a more specific answer depends on the library you are using.