I wouldn't consider it to be a DIY job.
Does it have a remote reservoir? If so, it'll contain pressurised nitrogen, so won't be home serviceable.
If it's just a basic shock with no reservoir, then you'll still have to completely strip it to get access to the end of the damper rod so that you can slip the new bump stop on. That will require a spring compressor then you'll have to remove the shock body cap to expose the circlip that holds the whole thing together. Even after all that, you'd still need to know the correct amount and grade of oil to refill it with.
If the shock is of an age that the bump stop has perished, then it's probably passed its best too. Time for a replacement perhaps?
Some places can refresh them, but I wouldn't think a stock Sachs was worth the effort.
Brook Suspension offer that service for £130 plus £18 for a bump stop
https://www.brooksuspension.co.uk/sa...epair-service/
Or you could try Mark Hammond Racing
http://www.mhracing.com/