应该很明显它替换UserName =部分，因为这正是您要求它做的。

为什么不替换未命名部分呢？类似于：

It should be obvious that it replaces the "UserName = " part because this is exactly what you are asking it to do.
Why not replacing the "Unnamed" part instead? Something like:

IO.File.WriteAllText(FilePath, IO.File.ReadAllText(FilePath).Replace("Unnamed", TextBox_NewUser.Text)

这很简单，见下文摘要

This is something really easy, see below snippet

'import System.IO
dim text as string = File.ReadAllText("test.txt")
text = text.Replace("some text", "new value")
File.WriteAllText("test.txt", text)