First of all, change both fields to Number - Integer
You haven't said where you want to do this.
Various methods
1. Set validation rules for the fields e.g. <=20
2. Use If statements in vba e.g.
3. Use intermediate if expression (IIf) to do the same thing in code or a queryCode:if rate>1000 then rate=1000
Also don't use Date as a field name as it is a reserved wordCode:Rate: Iif([Rate]>1000,1000,[Rate])