Time, supportive med. If kidney disease is reversible then the diagnosis will guide the therapy, for example, contrast nephropathy resolves over time, lupus required steroids and immunosuppresion. Diabetics require glucose control, hypertension control and ace inhibitors. This all requires support of the patient with dialysis if needed while healing. Call your doctor to discuss.
Kidney damage. There are very few treatments that heal kidney damage. However the kidneys have a capacity to heal with time. Acute kidney injury can often heal by stopping medications that damage the kidney (nsaids, IV dye, etc) and making sure you are well hydrated. If the damage is more long term (high blood pressure or diabetes), often you can only try to slow progression with BP control and sugar control.