Large time series file : need to replace #N/A with previous value( value in row above

I have a large time series file.  I have 200,000+  " #N/A ".  I want to take the data point in the row cell directly above.  

so if it's na in  cell d23.  i want to take d22's value and copy it into the n/a cell for the entire spreadsheet.

is there an easy way to do this in vba?  i've no experience using vba.

thanks