To Rename a Toolbox

In the Customize dialog ( File > Settings > Configuration > Customize ) , click the Tools tab.
Expand the User Tools section and expand the open DGN file.
Select the toolbox that you want to rename.
Do one of the following:

Click the toolbox a second time.

or
Right-click the toolbox and select Rename from the pop-up menu.

A rectangle appears around the toolbox's name.
Type a new name for the toolbox.