moving a field value to a second field

[Kevin]

I have stock Table that has a field, "QuantityInStock". i have also a
"StocktakeQTY" field that i fill in on staock take. How do i get the data to
copy from the "StockTakeQTY" field to the "QuantityInStcok" after i enter the
new figure and hit "Enter" or "Tab" and clear the "StockTakeQTY" field after
the data is transfered