To Delete a Cell from the Attached Cell Library

Select Drawing > Annotate > Cells dialog launcher.
The Cell Library dialog opens.
In the list box, select the name of the desired cell.
Do one of the following:

From the toolbar of the Cell Library dialog, click the Delete icon .

or
Right-click the cell and select Delete from the pop-up menu.

Since the deletion cannot be undone, an alert box opens, asking you to confirm that you want to delete the selected cell. If the selected cell is a shared cell in the active DGN file, its definition can be deleted from the active DGN file only if there are no instances of the shared cell in the active DGN file.
Click OK.