Mary,
If this is a one-time thing, you can put the exchange rate of the currencies
into a cell, select it and Copy, then select the money values to be
converted and do Edit - Paste Special - Multiply.
If it's an ongoing thing, you might want to have a separate column for U$D
amounts, deriving each amount from the associated Thai cell and the exchange
rate. It's a good idea to put the exchange rate into a cell, and name it
(Insert - Name - Define). I called it ThaiUSD in this example. Now use the
name in your formulas. If the exchange rate changes, you have only to
change the one cell.
=A2 * ThaiUSD
If you want to lock down the US values at a particular exchange rate value,
just select and copy the US column (the one with the formulas), Copy, then
with it still selected, Edit - Paste Special - Values. They'll no longer be
formulas and won't float with the exchange rate any more.