There are two diodes in opposite directions in parallel with the meter. This diode arrangement protects the meter no matter which way it is connected to the voltage source. When a diode fails, it will shunt the current; the current goes through the failed diode instead of the meter.
The replacement diode is part number 1N914/4148, a very common, low cost switching diode available at most electronic supply stores.
To replace the diode, open the meter by removing the two screws on the front panel. Be careful when handling the meter movement; do not let it fall accidentally. The two diodes are soldered between the terminals on the back of the meter movement. A failed diode usually has a blackened appearance. Remove the burned diode by clipping its leads. Make sure that the replacement diode is in the original orientation and then solder it into place.
If the meter needs to be used before the diode can be replaced, the failed diode may be clipped out. The meter will operate properly again, but is not protected from an overload.