I'm a little confused as to why you describe that your 'area' inherits from QGraphicsView, but then discuss QPolygonF. If you're using QGraphicsView, that implies that you have a QGraphicsScene and would be using QGraphicsPolygonItem.
However, with the assumption that the QGraphicsView is irrelevant here, QPolygonF is simply a class that inherits from a vector of points; QVector.
You should be able to iterate through the points and just move them wherever you want. You can get a point like this: -
QPolygonF poly; // assume it has been given a number of points
QPointF& point = poly[index]; // where index is the index into the QVector of points
Then you can move the point: -
point.SetX(xPos);
point.SetY(yPos);
Redrawing the item as a point is moved should not be a problem.