How can I pinpoint a character in a string and replace it - excel

Question

I am attempting to write a Marco to select the highlighted cell in the image attached (B11), locate the highlighted colon (20th character from the left) and replace it with a dot and loop this to do all 2500 rows in "B"
I am new to writing macros an would appreciate any help you can give me

Answer 1

Small showcase of how would you search for the last occurrence of ":" in Range("B11") and replace it with a "."
Sub replaceTest()
Dim val As String
Dim pos As Long
Dim rng As Range
Set rng = Range("B11")
val = rng.Value
pos = InStrRev(val, ":")
If pos > 0 Then
Mid$(val, pos, 1) = "."
rng.Value = val
End If
Set rng = Nothing
End Sub

Answer 2

Since your date times 18/10/2022 11:42:10:358 look to be text. I recommend to convert them into a numeric date time (so you can calculate with it and use comparisons which is greater or smaller).
Therefore you need to split the text up into its parts and turn it into a real numeric date using DateSerial and TimeSerial. Finally you need to calculate the milliseconds and add them.
Then you can use .NumberFormat = "DD\/MM\/YYYY hh:mm:ss.000" to format it as you like.
Public Function StringToDateTime(ByVal InputString As String) As Double
Dim InputDateTime() As String ' Split into DD/MM/YYYY and hh:mm:ss:000
InputDateTime = Split(InputString, " ")
Dim InputDate() As String ' Split into DD and MM and YYYY
InputDate = Split(InputDateTime(0), "/")
Dim InputTime() As String ' Split into hh and mm and ss and 000
InputTime = Split(InputDateTime(1), ":")
Dim RetVal As Double
RetVal = DateSerial(InputDate(2), InputDate(1), InputDate(0)) + TimeSerial(InputTime(0), InputTime(1), InputTime(2)) + InputTime(3) / 24 / 60 / 60 / 1000
StringToDateTime = RetVal
End Function
Public Sub Example()
Dim Cell As Range
For Each Cell In Range("A1:A5")
Cell.Value2 = StringToDateTime(Cell.Value2)
Cell.NumberFormat = "DD\/MM\/YYYY hh:mm:ss.000"
Next Cell
End Sub