"Flipping" or "mirroring" makes no sense in terms of a PCB. Mirroring a Footprint would trash the pinout of anything other than resistors, inductors and unpolarised capacitors. The only action that makes sense is to swap top for bottom or vice versa but that will still trash all the routing. Do Unroute All then select all in a layer and move it off the board. Select all on the other layer then swap layers. Select all from the other layer and swap that layer then move it back into place on the PCB. If you have parts on it one side of the board that makes the above simpler. Then re-route the PCB.

There is a extension can do this, but it only mirror the view [https://easyeda.com/forum/topic/Extension-User-Extensions-for-EasyEDA-Summary-9e065b68316f4491a3911dc6204be31e](https://easyeda.com/forum/topic/Extension-User-Extensions-for-EasyEDA-Summary-9e065b68316f4491a3911dc6204be31e)