Replace field contents.
In the 'records' menu on top there is a replace field content option (command = also does the job). In the window select replace with "calculated value" and just put a 0 or "". then hit replace. make sure you are showing all records as this command is issued only for the found records.
Be careful - the replace function is really powerful. And cannot be un-done. There is no Cmd-Z.
True, recommend duplicating the file and playing with the duplicate