This can be done easily by calculating the date in a new cell. Let's say you have your dates in column A. You can calculate the dates you need in column B with this;
I suspect however that you want to make the change in the cell where the date appears. Normally this is not a good idea because inevitably someone will ask you what the "real" date is supposed to be and that will be lost because you overwrote it.
But if you must then you would need to use a macro like this (again assumes dates are in column A
lRow = Range("A" & Rows.Count).End(xlUp).Row
For i = 1 To lRow
If IsDate(Cells(i, 1)) Then
Cells(i, 1) = Year(Cells(i, 1)) & "/" & Month(Cells(i, 1)) & "/" & 1
If you need to go the Macro way let me know the layout of your spreadsheet e.g.
which column your dates are in, what row they will start in, will there be any blank rows etc., and then we can adapt the macro to your requirements.