A bit of work.
Unhook the connectors, remove the whole thing from the game. Underneath if you still have the thin sheet metal spaces between brackets and playfield - save them and remember where they go.
Then remove the pins from the connector, then undo all the screws and the bookcase comes off the table.
Put on decals, then reassemble the whole thing.
Me, I'd clean the surfaces where they go, cut to fit, stick them on. And save all the taking apart and reassembly and F'ing stuff up in the process work.
LTG : )