Joyce, (Conifer, CO)
(For additional info regarding re-stringing day/night shades, see this other archived question.)
Replacing the complete assembly, however, is so much easier. Look closely at how they are hung. There will be brackets of some sort that attach the assembly inside or just above the window opening. Though attaching methods vary, rarely will the inside garnish trim need to be removed. If you’re handy with common tools, you should have no problem replacing them yourself.