If your turtle has developed a soft shell it is probably caused by one of two things.

It could be Metabolic Bone Disease (MBD) which is caused by a lack of calcium in the diet. If the condition is not too advanced it can be corrected by increasing the Calcium and Vitamin D in the turtles diet. This can be done by adding a supplement to their diet.

The other cause of a soft shell is shell rot which is usually caused by an infection, most commonly a bacterial infection. The infection then damages and weakens the shell. A vet would be able to advise you a treatment plan to hopefully correct the problem.

I highly recommend visiting a veterinarian to determine the cause of the problem. They will then be able to best advise the correct treatment approach.