Tip pitted. Smooth tip with a fine metal file; then apply flux to surface and melt solder on it.
Tip worn out. Most tips can be unscrewed. Then all you do is screw in a replacement tip. However, if tip refuses to turn, do not try to force it. Let serviceman make repair.
Electric iron wont heat. Heater has burned out. If you have manufacturer's instruction sheet, disassemble iron according to directions and put in a replacement heater. Otherwise, give iron to an expert for repairs.