If you are looking to mirror (flip) select traces you can use the RMB Menu "Mirror Geometry" via the Move or Copy command then you can change the traces to the new layer.
If you are looking to flip the entire board or a large circuit you can generate a Place Replica to perform this task. Once you generate a Place Replica of the components, clines and vias it will become a group that can be moved as one element, mirrored(flipped) to the opposite side of the board. This effectively mirrors the entire design with top side traces being moved to the bottom side, etc. Once it is flipped you can Disband the group so they are no longer related. Let me know If you haven't used Place Replicate before I can quickly go thru the basics.
Hope this helps,