Take a look at this code, you can use the mid$ to retrieve part of string AND to change it
Dim strTest As String
'A sample string
strTest = "abcdefghijklmnop"
'Look at it
'Change the 6 caracters starting at 5
'It will only use the 6 first caracters of the string after the =
Mid$(strTest, 5, 6) = "666666"
'Look at it, it changed!! :)