As with any other object, you edit your armature in Edit Mode Tab.
The set of bone editing tools is quite similar to the one for mesh editing.
One important thing to understand about armature editing is that you edit the rest position of your armature, i.e. its “default state”. An armature in its rest position has all bones with no rotation and scaled to 1.0 in their own local space.
The different poses you might create afterwards are based on this rest position. So if you modify it in Edit Mode, all the poses already existing will also be modified. Thus you should in general be sure that your armature is definitive before starting to skin and pose it!
Please note that some tools work on bones’ joints, while others work on bones themselves. Be careful not to get confused.
You can prevent a bone from being transformed in Edit Mode in several ways:
All bones can be locked clicking on the Lock checkbox of their Transform panel in the Bones tab;
If the root of a locked bone is connected to the tip of an unlocked bone, it will not be locked, i.e. you will be able to move it to your liking. This means that in a chain of connected bones, when you lock one bone, you only really lock its tip. With unconnected bones, the locking is effective on both joints of the bone.