[ms_access] SearchNull/Replace Query Help Requested

[Bud Blubaugh]

I have a table that has many null fields and I want to replace them
with say a zip code of 30512.  How do I write the query?
something like:
if zip_code - ISNULL then 30512   or something like that?
Can I use the search/replace window ??
Thanks,
Bud