The Replace () method does not actually change the contents of the string. Therefore, you need to assign a new value.
Example:
someString = "First Example"
someString.Replace("First", "Second")
// someString is still "First Example"
newString = "Hello World".Replace("Hello", "Hi")
// newString is now "Hi World"
Some examples: http://www.dotnetperls.com/replace-vbnet
Update:
From your recent comment, it seems like you want this:
TextBox1.Text.Replace("Hi", "temp").Replace("Hello", "HI").Replace("temp", "Hello")
. . , "" "" "" "", .