If rather than CHANGING blank values to be 0 you simply need to DISPLAY "0" in your forms and reports, then use the function Nz(), called Null-Zero. You can use this function in queries, reports, and forms.
For example, imagine you have a field called "Units". In the QBE grid (i.e., query design mode), change the field name in the grid from "Units" to this formula: Nz([Units],0)
In this example, the Nz() function translates to "If [Units] is null, blank, or 0, return a value of 0". If you wanted to display the phrase "Not available", the formula would be Nz([Units],"Not available"). In your case, you want to display "0".
NOTE: Nz() does not change the value recorded in the underlying table. It simply displays something else than what is there. If you want to actually change the values, you'd need an update query or VBA code within a form that changed blanks to 0.