You can manipulate bones through code, but it's likely easier to position a target bone and let IK do the rest. Check out the spineboy demo here (drag the red circles around):
Spine: Demos
You can look at the spineboy example project to see how it is rigged and it is explained here:
Spineboy example
Anyway, to move the target bone around you can use spine-unity's SkeletonUtility. This allows you to have a bone follow a GameObject (which likely is what you want), or a GameObject follow a bone.
