On Basecamp I would like to change the starting point of the circuit

Select the Move tool and hover over the point until it turns into a black circle, you can then click and drag the point.

Hello. Sorry I don't understand. What do you mean by MOVE TOOL ? (I have a french version) . 

I do not want to drag the point but only in a loop circuit I want change de starting point on a other point.

Ah ok, I thought you wanted to move the start point. If you want to do what I think you now want then you'll need to split the route into two at the appropriate point and then rejoin the two parts in the correct order.

Thank you very much for your answer. Too complicated handling, I will continue by using EditGPX which with a single click allows to modify the file. Pity.