You need to use an expression...
such as:
In design mode of your query, in the "field:" row select the next empty column (of the fields to be returned or viewed in your query)...
add....and replace the generic table and field names to yours.
MyExpression: IIf([tblName]![fieldName]<0,0,[tblName]![fieldName])
be sure to use the dbl II in the IIf statement