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.