1993 Nissan Sentra Ventilation

What to replace is what you pay a mechanic to diagnose. In this case, there's two likely suspects. The first is the switch assembly that may have burned or overheated contacts. You have to check the connector terminals too for signs of overheating. Typically only the lowest speed works, regardless of which setting the switch is on, but that depends on how and which part failed.

The second is the resistor assembly that the switch is connected to. Often when those fail, only the highest speed still works. When multiple speeds work while others don't, rather than suspecting the resistor, suspect the terminals plugged into that assembly.