Top burner gas valves are fairly easy to replace.  With the top plate (bull nose) removed you can access them easily (part of the frame runs across the top of them).  If you are replacing all or just one, the burners must be removed if they have a slip type orifice.  If there is a tube type burner valve they need not be removed because you can disconnect the tube from the burner valve and move it out of the way. 

In either case when the burners or the tube is disconnected take a wrench and screw out the valve being replaced (in some cases you may have to remove a pilot adjustment valve to be able to turn the burner valve by it).
 
Once the old valve is removed, put some gas thread sealer on the treads of the new valve and screw it back into position, being careful not to over tighten.  Reassemble the unit except for the top plate.  Turn the gas back on and light the pilots (this may take a few minutes because the gas must first force out the air in the manifold).

Once the pilots are lit, turn on the first burner.  There may be no gas flow because the orifice on the new valve may be closed.  Using two wrenches, one to hold the valve the other to turn the orifice, open the orifice to allow gas flow (this may take a couple of turns).

Once the burner lights, continue to adjust the orifice until there is a nice blue flame.  Adjust until you have as little of a yellow flame as possible. If you feel there is still too much yellow flame you may need to adjust the air shutter located on the end of the venturi.  Once you have produced the bluest flame you can get, you are done.  You can reassemble your range and cook to your heart’s content.

Make sure you have the right orifice for the type of gas you are using!  Natural gas and LP gas require a different sized orifice.  If you are having problems with the flame on the burner, check the number on the orifice.  The larger the number, the smaller the hole.  Large number orifices are for LP gas.  Smaller numbers are for natural gas.