Download extension.zip from Releases and extract the files to your disc. Open EasyEDA, go to "Advanced" > "Extensions" > "Extensions Settings ..." > "Load Extension..." > "Select Files ..." > Select all files from the extracted directory > "Load Extension".

A "ToggleView" Menu should appear in the main menu in PCB view.

Click ToggleView to toggle the current PCB editor view between regular and flipped view.

In the Flip on Layer menu select the layers which, when activated, should always be flipped automatically. You can always use ToggleLayer to override this.

The X ruler is not flipped correctly

This is just a hacky workaround until EasyEDA implements it correctly. It basically applies the style transform: scaleX(-1) to the editor to flip it in X direction. To still be able to interact (click) with the flipped view with your mouse correctly, all mouse events also need to be catched and flipped in x direction.