You can modify a block definition using several methods.
There are several methods for redefining a block definition. The method you choose depends on whether you want to make changes in the current drawing only or in a source drawing also.
The Block Editor provides the easiest way to modify a block in the current drawing. The changes you make and save in the Block Editor replace the existing block definition, and all references to that block in the drawing are immediately updated.
An alternative way to modify a block definition is to create a new block definition, but enter the name of the existing block definition. You can insert and explode an instance of the original block and then use the resulting objects in creating the new block definition.
Updating a block that was created in another drawing and then inserted into the current drawing does not trigger an automatic update (unlike inserting xrefs). To update a block that has been updated in another drawing, you must reinsert it.
Block definitions created in your current drawing by inserting a drawing file are not updated automatically when the original drawing is modified. You must insert the drawing again to update a block definition from the drawing file.
Inserting a block using DesignCenter does not overwrite an existing block definition. To insert a block definition that has been updated in a block library, for example, use WBLOCK to save the block as a separate drawing. Then, insert the drawing to overwrite the outdated block definition.
The block definition remains in the drawing, even when all references to that block are erased. In order to remove the block definition, you need to purge it.