How should I replace potassium loss?

With oralsupplements. This should be done under the guidance of your doctor as potassium is potentially dangerous.
Depends. The physician who diagnosed your low potassium levels should review potassium replacement options with you. Certain medical conditions cause loss of potassium. Those conditions must be corrected for the potassium level to normalize. Some medications cause wasting of potassium. Dietary adjustments and medication supplements may be part of your therapy. Review with your physician the best overall course of treatment for your specific condition.