The scenario described on your page re lilac trees described my problem exactly. However, I cannot reach the top and it is one section of the tree that is obviously dying. Do I need to cut the top and ALL the dying section or will an overall cutting back do it for the dying section? It is one entire side of the tree . . .
Yes you need to cut all of the dying section back to healthy wood. You may want to reduce the overall size of the tree to keep it in proportion after removing the dying branches. The master gardener’s post below gives a very good description of how to prune the lilac and I would also recommend watching the University of Maine Cooperative Extension video. I have added this link.
https://www.torontomastergardeners.ca/askagardener/lilac-pruning-2/