I wonder how to tranform e.g. a rectangular selected area? When I use the "transform a layer or selection" tool it immediatly starts to transform the image. But I just want to change the selection by dragging it. Is that impossible?

Thanks in advance for your help!

You can move a selection with the "transform a layer or selection" tool by clicking inside the selection and dragging. Or you can use the "move a layer" tool while a selection is active.

But this already transforms the slected part of the image. I just want to transform the selection.

I'm sorry, but we still need to implement a way to modify selections directly. You can create a local selection mask out of the global selection and then modify that with all the available tools, though.