Function doubleMe(d)
    Application.Volatile
    doubleMe = d * 2
End Function

Public Function doubleMe(d as variant)
    doubleMe=d*2
    end Function

Public Function doubleMe()
    Application.Volatile
    doubleMe=Worksheets("Fred").Range("A1")*2
    end Function

Application.Calculation = xlCalculationAutomatic

Application.Calculation = xlCalculationManual

Public Sub UpdateMyFunctions()
Dim myRange As Range
Dim rng As Range

'Considering The Functions are in Range A1:B10
Set myRange = ActiveSheet.Range("A1:B10")

For Each rng In myRange
    rng.Formula = rng.Formula
Next
End Sub

Public Sub UpdateMyFunctions()
    Dim myRange As Range
    Dim rng As Range

    'Considering The Functions are in Range A1:B10
    Set myRange = ActiveSheet.Range("A1:B10")

    For Each rng In myRange
        rng.Formula = rng.Formula
    Next
End Sub