Have array of Unit; Unit is base Class; How to access derieved class functions???

To access the functions of the derived class you need to cast your object. Hopefully the pretend methods below will illustrate the point. I've written this in c# as you didn't state a language preference. I've used an array, simply because you mention one in your question. Also, I've specified all types explicitly (rather than using var keyword) as it's hopefully clearer at this stage.

Assume a base class called Unit, and a class derived from this called SpecialisedUnit which defines the SpecialAction method.

 // if you know unit is definitely of a derived type then do this...
 void OperateOnUnits(Unit[] units)
 {
   foreach(Unit u in units)
   {
     ((SpecialisedUnit)u).SpecialAction();
   }
 }
 
 // if you don't know if unit is definitely of a derived type then do this…
 void OperateOnUnits(Unit[] units)
 {
   foreach(Unit u in units)
   {
     SpecialisedUnit specialUnit = u as SpecialisedUnit;
     if(specialUnit != null)
     {
       specialUnit.SpecialAction();
     }
   }
 }