The Exists method is used to determine whether a key already exists in the specified Dictionary. Returns True if it does and False otherwise.

The following code uses the Exists method to see if the key with the value of 'g' exists in the Dictionary object and, if it does, changes it's corresponding item value.

Code:
<%
dim guitars
set guitars=CreateObject("Scripting.Dictionary")
guitars.Add "e", "Epiphone"
guitars.Add "f", "Fender"
guitars.Add "g", "Gibson"
guitars.Add "h", "Harmony"
If guitars.Exists("g") Then
   guitars.Item("g") = "Guild"
End If
%>

Output:
"Epiphone"
"Fender"
"Guild"
"Harmony"