I'm not sure there is anything to save. I suspect we are talking rot and pretty extensively. Get a new plant and try again.

The soil in those pots looks dark and soggy. Cactus and succulents should NEVER be in soggy soil. You could also do with another soil mix, I use a 50-50 mix of potting soil and pumice, you would do well with a 50-50 mix of potting soil and perlite.

For your next plant: definitely give it more light, as these guys are looking etiolated. Also,

VERY IMPORTANT: Do not water on a schedule. Do not buy a wetness meter. Listen to your plant!
Use a toothpick (for small pots) or wooden dowel and insert it into the soil. When you pull it out, it should be completely dry if your plant is thirsty. If it comes out wet and covered in clumps of wet soil, DO NOT WATER.

When the soil is dry, water thoroughly until water comes out of the drainage hole.

Watering too frequently (especially in soil that is water-retentive) is the fastest way to kill succulents, as the damage is usually not something they can recover from.