Two tasks, same macro?

I want to put a value in a single cell - A1 is going to be (say) 5.

Sub Macro()
Cells(1, 1) = "5"
End Sub

Now, if A1 already contains the value 5 I want to change it to (say) 0 using the same macro.

Sub Macro()
Cells(1, 1) = "5"
If Cells(1, 1) = 5 Then
Cells(1, 1) = 0
End If
End Sub

Clearly doesn't work as the value 0 is always returned.

I want to link the Macro to a button such that the first click returns 5, click again returns 0, click again returns 5, and so on.....

How can I do this please ?

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Sub Macro()
    Range("A1").Value = IIf(Range("A1").Value = 0, 5, 0)
End Sub

I may be losing the bubble as to what you're trying to do, but maybe this:

Sub Macro()
    Range("A1").Value = IIf(IsEmpty(Range("A1").Value), "=B1+B2", vbNullString)
End Sub