if the seats are merely saggy, wet them thoroughly & dry upside down in the sun - it shrinks them up a bit so no more sag. Otherwise there are firms that sell replacement wicker to re-weave & replacement 'splines' -the piece that goes around the edge to hold the woven part in. Try googling 'wicker repair supplies'. If they are rush seats they CAN be repaired but a friend who did one said it really hurt her hands & she'd pay someone to do it next time or replace with a leather or plywood seat instead!
Hi Angela, perhaps this will help. https://www.diynetwork.com/how-to/make-and-decorate/decorating/how-to-repair-wicker-furniture
Wicker is so beautiful! Take a look at these ideas for fixing your wicker furniture!
This may be a simple quick fix: https://www.youtube.com/watch?v=TNVsW77bGyA