If you plan to just add new shingles over your existing roofing shingles then you don't need any Ice and Water Shield because it will do nothing to protect your roof.
Ice and Water Shield needs to be installed on the OSB or plywood roofing sheathing to protect that part of the roof.
But first you need to apply the roofing felt to the OSB or plywood roofing sheathing and then at the eves of the roof apply the Ice and Water shield and then apply the shingles.
If you want to install the Ice and Water Shield you need to remove the shingles and then apply the Ice and Water shield and then install the new shingles.
If you're putting on a new roof it's always best to just remove the old shingles first and then apply the new shingles because you get a better roofing job.