if you're referring to the white bodied speedo/tacho, then they are a bit of a pain to do, but do-able
first you need to strip them out of the holders/black casing, then get all the [usually] green silicone or whatever it is off. be careful doing this, because there's a notch in the white casing that aligns the speedo face, so you don't want to knock that off. once it's loose, pull all the inside workings out and put it safe, where it won't get scratched. the yellow lens is impossible to get out atm
when you get it out you'll see why. what i usually do is sit the white casing on the floor and aim a heat gun at the middle of the lens until it starts bubbling and melting, then prize it out with a strong screwdriver, while it's soft. it goes hard again very quickly, so you might have to have a couple of goes at it. be careful not to heat the white outer casing or you'll distort it. get some 2mm acrylic discs, cut to size - somewhere round the 80mm mark, but you'll have to check. i use this bloke:
https://www.ebay.co.uk/itm/2mm-Acrylic- ... 1438.l2649" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false; about £9 for 5 discs. get cast acrylic, not extruded. glue the lens on, after removing the backing paper and cleaning the lens. i use technicoll polychloride glue, which is for hard plastics. it takes several hours to harden though, but once it's hardened, it's STUCK. rebuild clock and make sure the cut-out is on the notch. you need some good silicone sealant to fix the metal base plate, or use gorilla glue - very sparingly, then the silicone over the top. leave for several hours to go off. once all hard, peel the top backing paper off the lens, give it a wipe and pop back in the black body. bit of a long, drawn out process, but once the clocks are back in they look good - and you'll be able to see how fast you're going
i've done about 20 sets like this and it really makes a difference to the look of the clocks.
cheers, dd.
ps, that technicoll glue will also mend plastic sidepanels and sticks them perfectly and strongly.