Easibleed, connected to the spare, at approx 15/20psi, or regulated from a compressor to the same psi.
So easy, just sit there with a plastic screw top bottle,(hole in the cap that the plastic hose goes through, with a small cable tie on the underside to keep it in place,) wire hook on the bottle, then just open and shut the bleed nipple, throw the old dirty away periodically, until the nice new fluid flows through.
Got a tapered plastic adaptor, that fits, and seals most of the nipple sizes whilst bleeding, minimum mess and fuss?
Normally clamp off the hose initially, remove the nipple, clean the threads, and apply a liberal amount of brake seal grease on the threads, to facilitate easy opening next time.
Finally, small pieces of plastic tube, cut to size, pushed over the nipples, filled with rubber grease, and then plugged, so road muck and salt is kept well clear of the nipples in future.