How can I change the Start Point of a vector?

Select the vector and press N, the short-cut key for Node Editing.

If a node already exists where you want the Start Point to be, you can either hover the mouse pointer over it, right-click to open the Node Editing menu and select Make Start Point OR hover the mouse pointer over the node and press P, the short-cut key to Make Start Point.

If a node doesn't exist where you want the Start Point to be, hover the point of the mouse cursor over the spot you want the Start Point to be and press the short-cut key P. This will create a new node at that spot and make it the Start Point, all in one operation.

Note: The Start Point on an open vector has to be at one of its ends but can be anywhere on a closed vector.