Burner heads usually become plugged over time. There are two screws that connect the head to the venturi. It is a good idea to use some kind of rust dissolver on the screws to loosen them. Once the screws are out you can install the new head, and always remember to replace the gasket as well.
Some burner heads, most notably Southbend, are one piece and do not have a separate venturi. In these cases the whole piece needs to be replaced.
One alternative to replacing the head is to get a drill bit the same size as the holes in the head and drill out the grease build up in them. However, this can sometimes alter the flame pattern.