Find and replace second instance

I believe you need a macro for that.
Try selecting the cell(s) containing the formula and run this macro.

Sub SearchAndReplace()
    Dim c As Range, s1, s2, n, j, k
    s1 = "12"   ' Text to search
    s2 = "13"   ' Text to replace with
    n = 2       ' Nth instance to replace
    
    For Each c In Selection
        k = 0
        j = 0
        Do While InStr(j + 1, c.Formula, s1)
            j = InStr(j + 1, c.Formula, s1)
            If j Then k = k + 1
            If k = n Then
                c.Formula = Left(c.Formula, j - 1) & s2 & Mid(c.Formula, j + Len(s1))
                Exit Do
            End If
        Loop
    Next
End Sub

Maybe, but I don't know how.