To modify a stored entity object, retrieve it, update it, then put it back to the entity store:
SimpleEntityClass sec = sda.pIdx.get("keyone");
sec.setSKey("skeyoneupdated");
sda.pIdx.put(sec);
      
                Note that because we updated a field on the object that is
                a secondary key, this object will now be accessible by the
                secondary key of skeyoneupdated instead
                of the previous value, which was skeyone
        
Be aware that if you modify the object's primary key, the behavior is somewhat different. In this case, you cause a new instance of the object to be created in the store, instead of replacing an existing instance:
// Results in two objects in the store.  One with a
// primary index of "keyfive" and the other with primary index of 
//'keyfivenew'.
SimpleEntityClass sec = sda.pIdx.get("keyfive");
sec.setPKey("keyfivenew");
sda.pIdx.put(sec); 
      
                Finally, if you are iterating over a collection of objects
                using an EntityCursor, you can
                update each object in turn using
                EntityCursor.update(). Note,
                however, that you must be iterating using a
                PrimaryIndex; this operation is not
                allowed if you are using a
                SecondaryIndex.
        
                For example, the following iterates over every
                SimpleEntityClass object in the entity
                store, and it changes them all so that they have a
                secondary index of updatedskey:
        
EntityCursor<SimpleEntityClass> sec_pcursor = sda.pIdx.entities();
for (SimpleEntityClass sec : sec_pcursor) {
    sec.setSKey("updatedskey");
    sec_pcursor.update(item);
}
sec_pcursor.close();