Golfinray -
Before messing with data, make a backup...
To change existing data, you need an update query. You will add only the
number field you are trying to change to the design grid. In the Criteria
row, enter a zero (0). In the Update To row, enter Null. Switch to
datasheet mode, and you should only see the zeros (this is showing the
current value of the records that will be changed). Switch back to design
mode and run the query (red exclamation point button).
You may want to fix the source of the zeros if you can. If your table shows
a default value of zero for that field, you may want to remove that default
value. If your data comes from a form, then you can change zeros to nulls
with a BeforeUpdate event on that numeric field. If it is imported data,
then you may need to run the query each time you import.
Hope that helps!