Beau has written a nice simple article on his blog Rndr4Food about updating blocks in AutoCAD.

"Do you use acad blocks? Do you use the same blocks in multiple drawings? Ever need to update the blocks?

If you answered yes to any of the above questions then this posting is for you. I recently gave a refresher/updated materials course for a few AutoCAD users and part of this involved different methods of using blocks and redefining existing definitions in different drawings. If you need to accomplish this here are a few examples of ways to accomplish this"

Will render for food...: AutoCAD Blocks