Modify panel > Tool buttons > Make Unique
Modify panel > Right-click an instanced modifier in the stack display. > Make Unique
Make Unique lets you convert an instanced modifier to a copy that's unique to the current object. A modifier is instanced when it is applied to multiple selected objects. You can also use Make Unique to make instanced or referenced objects unique.
Procedures
Example: To use the Make Unique options with an instanced modifier:
Create a cylinder with some height segments, and then use Shift+Move to create three copies, resulting in four identical cylinders.
Select all four cylinders, apply the Bend modifier, and adjust the Angle setting just enough to see the results of the bend.
You now have four cylinders with a single instance of a Bend modifier applied to them.
Choose Edit menu > Hold.
This lets you return to the current state of the scene at any time without using Undo.
Select two of the cylinders and click Make Unique.
Choose Yes in the resulting dialog.
At this point, the two selected cylinders each have unique Bend modifiers, while the remaining two cylinders share the original Bend. You can see this by selecting each cylinder and changing the Bend Angle setting.
Choose Edit menu > Fetch, and answer Yes.
Select two cylinders again, and click Make Unique.
Choose No in the resulting dialog.
The two selected cylinders now share an instance of a Bend modifier, but it's a different instance than that shared by the other two cylinders. Again, you can see this by selecting each of the cylinders and changing the Bend Angle spinner. You can also turn on Show Dependencies in the Views menu to see the relationship between the four cylinders and the Bend modifier.
Example: To use Make Unique with an instanced object/modifier combination:
Create a cylinder with some height segments.
Apply a Bend modifier and adjust the Angle just enough to see the results of the bend.
Use Shift-Move with the Instance option to create an instanced object/modifier combination.
Change the Bend Angle setting on one of the instances to demonstrate that the cylinders and modifiers are truly instanced.
Both cylinders bend.
Select one of the instances, and then, in the modifier stack choose either the Bend modifier or the cylinder itself.
Click Make Unique.
When you change the Bend Angle setting or cylinder base parameters for one of the objects, the other doesn't change.
Note: When you instance an object/modifier combination, all duplicates are instances of a single master node containing the original object and modifier. In such cases, you cannot selectively make the object or its modifier unique. Clicking Make Unique for one or the other, makes both unique.
Interface
Make Unique: Detaches objects and modifiers (and combinations) logically from the master node of which they're instances or references.
Go to the object or modifier level in the stack for an instanced or referenced object or modifier (respectively), and click Make Unique.
For instanced object/modifier combinations, in the modifier stack, choose either the modifier or the object itself.
Make Unique is unavailable when a selected object is not an instance or reference, when it doesn't contain an instanced modifier, or when all objects in the selection don't have an instanced modifier in common.
Making New Instances
When you click the Make Unique button for a selection of two or more objects, a dialog appears that asks: Do you want to make the selected items unique with respect to each other? This gives you a choice of how you want to make the objects unique.
Yes: Makes the instanced modifiers assigned to the objects in the selection unique for each object in the selection.
No: All instanced modifiers assigned to the objects in the selection remain instanced across the selected objects, but become unique from other objects not in the selection.