Not sure how to do that in Render Window, but im Raytraced mode you can click the sample count in the lower right bottom, in the HUD. It should turn into a field where you can input the new sample limit.
For the Render Window it will be difficult, especially if the end already has been reached. The render session, and the Cycles instance that did the rendering already has shut down and cleaned up at that point. Maybe a command to increase for a still running session, but not for one that has ended.