Top rack was falling down and /or going out of it's track. Noticed that the rollers were 'broke off'.

One side at a time I removed the plugs at the end fo the rails - this was the hardest part... needed to twist them while squeezing them to get them out of the end of the rail. Once off, easy to unclip old roller assembly, remove, replace and re-clip new.
One side seemed to be easier to replace the plug onto the rail - only used screwdriver to push plug back in place once in the right position.