Choose the Delete Regions command 
.
Select the edges that surround the region you want to delete.
On the command bar, click the Accept (check mark) button.
Select a face that lies inside or outside the region of edges you defined.
On the command bar, click the Preview button. All the faces that lie on the side of the face you selected in step 4 highlight.
On the command bar, click the Finish button.
Tip:
You can delete regions on the design model or you can delete regions to create a simplified version of the model in the Simplify Model environment.
The regions you delete in the Simplify Model environment are added to the Simplify section of PathFinder.
You can use the commands on the shortcut menu to edit, delete, suppress, and rename the simplified features you define.