If someone has renal failure, how can they be treated for hypernatremia?

Hypernatremia. Reduce salt intake and balance that against increasing water intake. The caveat here is where is the patient in the continuum from end-stage renal disease being managed medically to being on dialysis? Each scenario has different answers to reducing sodium in the blood. Also important to know is what caused the kidney failure in the first place because treatment has to be tailored to the disease.
With fluids. Patients that are dehydrated could present with both, the numbers of BUN and creatinine and the sodium level elevated. Drs call any increase in BUN and creatinine as renal failure which could be of recent origen or chronic (more insidious onset). Some patients have chronic renal insufficiency and suddenly get worst due to dehydration. Treatment is intravenous fluids first.