"I am looking at the inverted bottom side silkscreen.
How can I "re-invert" it that the Ref. design would be readable?"
You need to use the mirror command. The mirror command, like many other operations, works on the selection. So you first need to select the data.
(1) Select the layer to mirror.
There are many ways of doing that:
(1.b) Right-click on the layer in the layers toolbar and pick "Select/New"
(1.c) Make sure the layer is the current layer and "Only" is checked, then press "V" (shortcut for "Select/Global/New").
When done, to deselect everything use "Select/Clear Selection" (shortcut "Ctrl+U").
(2) Mirror the selection: "Edit/Edit Selection/Mirror". You can pick either a mirror on the X axis or Y axis and you also have to specify the point around which to mirror.