Well I am guessing that it is the same on a 124 as a 126. There are actual tools for this (T handled rods with a 90 degree bent in ends) that you stick in the side of the cluster and pull out. I just used a coat hanger. If you still don't understand, do a search on this...many people have described this procedure, some with pics.
As far as the odo not working, I doubt you will be able to fix it. Your best bet would probably be to send the speedo into Palo Alto Speedometer (www.paspeedo.com
), or any other competent speedo shop for that matter (that is just the one I have heard most about).... to have them fix the odo and/or refurbish your speedo.