It has a "Bayonet" type socket with 2 retainer pins.
Here is a direct replacement: http://www.tubesandmore.com/products/P-47
Unplug the amp.
Use the eraser end of a #2 pencil.
Gently push it into the hole until it stops.
Then, push it a little further, the bulb is spring loaded.
Turn the pencil counter clockwise (toward your left hand).
Once the retainer pins have been properly aligned, you should be able to use tweezers or small needle nose pliers to get it out.
You could probably even lay the amp on it's face (gently) after you've aligned the pins, it might just fall out.