im guessing its in an ea-ed speedo. easy enough to pull the instruments out, undo the screws holding the lens/black fascia on and unclip. on the back of the white housing are 4 screws, directly behind speedo, undo them and the speedo section will pull out. refit another, put the screws back in clip back together, refit and your done.
on ebay and here from time to time people are selling the new odometer cogs that u need to fix a broken odometer too. they are a little fiddly to fit, but will last a lot longer than used ones.
ps if u are buying a used one from wreckers make sure the odometer hasnt been reset (showing 000) as this could be broken too
and remember dont reset your odometer when moving as this strips the gear.