Well, first you'll have to really wash the cement. It'll take more than just water to get the smell out of the cement so that your dogs don't smell that they went there before and will be attracted to go there again. You could soak it in Nature's Miracle or some other pet cleaner, then scrub it and spray it off with water.
Then you'll have to prevent your dogs from having the opportunity to go on the cement. Walk out into the grass with your dogs, and if they even walk on the concrete before they've pottied completely, quickly move them back onto the grass. You might even have to put them on leashes to keep them out in the grass with you. It will probably take at least a week, most likely longer, before they choose to go on the grass.