In the previous reply that probably should be Image>Rotate>Flip Layer Horizontal.
Of course you will have to replace the original flag that is still visible. Hopefully you can do that by cloning or copying adjacent area. In this example (flag waving on left side of pole) I copied the flag to a new layer, flipped it, and moved it to the right side of the pole.
To replace the old flag I selected a portion of the sky:
Copied/pasted this selection to a new layer and moved it to cover the old flag:
Since the sharp edges of the patch were visible I use the Clone tool (with a large soft brush size and the "Use All Layers" option) to clone adjacent sky over the edges. Here is the result:
