I'm not sure what is meant by "flipping" the selection, but I did the following:

1. Make my selection
2. Copy the selection
3. Create a new layer
4. Paste my copied selection into the new layer
5. Layers >> Flip >> Horizontal/Vertical

Selection has now been "flipped". Took all of 3 seconds. And that's the first time I've ever opened the software.

Read the manual, it's a good place to start learning ~ ulillillia