Replace an Occurence in a String

Does anyone know how I can fix this:

I'm trying to use some sort of loops, either nested loops perhaps to get VB to go through a single string;

For example - S = "His name is Josh Howard. Josh is a great athlete. "

and I want the program to be able to find the word "Josh" and replace it with "Joe" instead. I'm trying to break the S string into three substrings, the first one is the text before the occurrence, the 2nd is the word "Joe", the 3rd being the text after the change. Make them equal S and continue the search.

Any ideas.... I solved it using the replace method so yes i know, but I figure you have to be able to solve this with just strings, the indexOF, and Length methods.

Here is my loop section of the code:

S = txtinput.text ' the inputted string (His name is Josh Howard. Josh is a great athlete)

LenS = S.Length

For p = 0 To LenS - 1

'S2 = S.Substring(i, LenS)
AVP = S.IndexOf("Academic Vice President")
' N = S.Substring(AVP + 23, LenS - 1)
S2 = S.Substring(i, AVP)
i = AVP + 23
'i =
S3 = S2 + S1
'S3 = S3.Substring(AVP + 23, )

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