First of all, if you're using AutoCAD 2002 or above, try double-clicking a block. That should bring up the refedit command (unless the block has attributes... more on that in a sec.) Select the items you want to change, change them however you want, click the 'Save' button on the toolbar that should pop-up with that command and all of the other blocks in the drawing will be updated.
If the block has attributes you can do the same thing with the refedit command except that it's easier to just run it from the command line. Double-clicking a block with attributes will bring up the BattMan command (Block Attribute Manager) which allows you to modify the attribute properties of a block.
If you are talking about a mass update of one attribute value to another value then you can use -attedit. Say, for instance, that you have a bunch of terminal blocks on a drawing and the all are number A-1, A-2, A-3, etc... and you need to change them to B-1, B-2, B-3, etc...
Use -attedit to perform a mass change in much the same way that you use chgtext or CT. When prompted 'Edit attributes one at a time? [Yes/No] <Y>: answer NO then take the default for the rest of the prompts. Select all of the blocks you want to change and hit ENTER. Type the string you want to change (in our example 'A-') and hit ENTER. Type the new string (in our example 'B-') and hit ENTER.
Voila!