It's dropping leaves because it's being overwatered. Succulents need to let their roots dry out or they'll absorb too much water which will make cells in the leaves swell up and burst, killing them. In severe cases, the roots will start to rot which kills the plant. It's sort of counterintuitive because people will see the leaves wilting and will try to water the plant more to perk it up, which only makes things worse.

Fortunately the center still looks healthy, so saving it will be easy: just slow way down on the watering. Water it heavily, but only water it when several of those center leaves start to feel a bit soft to the touch and/or wrinkle slightly. Depending on the plant that could take anywhere from a bit under 2 weeks to well over a month. Always leave the soil completely dry all the way through for at least a few days between waterings. The absolute worst thing you can do is give it tiny splashes of water every few days, that will inevitably cause root rot followed by tragedy.

And also make sure that the pot(s) it's in have drain holes on the bottom. Standing water is a death sentence for succulents because water will pool inside the pot and keep the roots wet.

You should also move it to the brightest spot you can find so that it can get enough light to stay happy and grow strong new leaves. A clear south facing window is ideal if you live in the northern hemisphere. Good luck!