A lot of it will depend on the INR and what therapeutic range you want it to be.
Obviously if the patient took an overdose and is not even supposed to be on it, Vitamin K is beneficial - however, in an emergent setting a transfusion of FFP (fresh frozen plasma) is indicated.
In patients that are supposed to be on it and just happen to have too high of an INR - a lot depends on whether the patient is symptomatic or not. If not symptomatic (ie, bleeding uncontrolled), then just observe. If symptomatic, then as above for emergencies.
There is a guideline out there for what you do for a given INR, however - remember each person responds to coumadin
differently depending on numerous factors including dietary intake. Consequently relying on that chart may not give the correct results.