You can redefine all instances of a block within the current drawing. To redefine a block that was created in the current drawing, you create a new block using the same name. You can update all the blocks in the current drawing by redefining the block. If the block was inserted from a separate drawing file that was subsequently updated, reinsert that block to update all other instances in the current drawing.
There are two methods of redefining block definitions, you can choose to modify block definition in current drawing or in original drawing.
Modify a Block Definition in the Current Drawing
To modify a block definition already inserted into the drawing, you should first create a new block definition with a new name to replace the existing block definition, and all the references to that block in the drawing are immediately updated to reflect the new definition.
To save time, you can insert and explode an instance of the original block and then use the resulting objects while creating the new block definition.
To redefine a block in the current drawing
Choose Draw > Block > Make.
Type the name of the block you want to redefine, and then click OK.
Specify the insertion point for the block.
Select the objects for the block, and then press Enter.
In the dialog box of "The block already exists, Do you want to redefine it?", click YES.
Command line BLOCK
The block is immediately redefined, and all instances of the block in the drawing are updated.
TIP To restore the original objects to the drawing while retaining the new block, click Retain.
Update a Block Definition That Originated from a Drawing File
To modify the original drawing directly before inserted into the drawing, blocks inserted into current drawing are not updated automatically unless the block definition from the drawing file is updated using INSERT command.
Remove Block Definitions
Too many block definitions in a drawing file may affect the drawing size. To reduce the size of a drawing, you can remove unused block definitions. Erasing a block reference from a drawing may delete the block reference but retain the block definitions in the block definition table. Using PURGE command can help removing unused block references from your drawing in order to reduce the drawing size.
You should erase all references from a block before you can purge the block definition.
Disassemble a Block Reference (Explode)
To modify one or more objects within a block separately, you can use explode tool from Modify toolbar, and restore this block after you have modified these objects using one of the following method:
You can automatically explode block references as you insert them by selecting the Explode option in the Insert dialog box. Thus, the block reference will be automatically exploded with each object inserted into the drawing separately.
To explode a block
Choose Modify > Explode.
Select the block.
Command line EXPLODE
BLOCK: Defines multiple objects as a whole to create block definition
EXPLODE: Breaks a compound object (such as block, polyline and so on) into its component objects
PROPERTIES: Displays the Properties dialog box, in which you can changes the relevant properties of existing entity
System Variables Reference