use a dremal with a rotozip bit to cut out the old ones. Take the new ones and place them where they will be installed. If there are gaps from where the old holds were cut out, then you will need to either build that area back up then install new footholds or install new footholds and then lay glass over the gaps. I would also refoam first if you go this route, let the new foam come through the gaps, once dry, sand smooth and lay glass cloth or matting across the old void where the foam has been smoothed out, that way you have something to lay the glass up against.......
good luck