search for characters within a file and overwrite them

I want to overwrite certain characters within a file
for example if a file contained the characters "FT"
I want to search for the occurrence of "FT" and overwrite it with "HT"

Does anybody know of an API or function which will carry out this task

any help is greatly appreciated

Code:

---

Dim sText As String

Private Sub Command1_Click()
    Open "C:\MyFile.txt" For Input As #1
        sText = Input(LOF(1), 1)
        sText = Replace(sText, "FT", "HT")
    Close #1
End Sub

Private Sub Command2_Click()
    Open "C:\MyFile.txt" For Output As #1
        Print #1, sText
    Close #1
End Sub

---