A family of Microsoft relational database management systems designed for ease of use.
If you want to restrict the update to those rows where the Item_PO# is one from a specified list of values you can use the IN operator in the WHERE clause like this:
UPDATE YourTableNameGoesHere
SET Item_PO# = 0.01
WHERE Item_PO# IN(0.98, 0.05, 0.04);
Just extend the value list to include whatever values you want to be updated. It would be pointless to include 0.01 in the value list of course, as rows with that value do not need to be updated.