Perimeter editing 
Enter edit mode 
Select the plane that you want to edit:

Press "E" or click on the "Edit perimeter" button:

Move perimeter points 
Click on the point that you want to move/edit:

Move the point to the desired position:

Repeat with any other point:

Finish edition 
You can finish editing the perimeter by pressing "S" or clicking on the "Finish" button:

Add new perimeter points 
Select the point that you would like to add in between two existing points:

Move the new point to the desired position:

Delete perimeter points 
Select the point that you want to delete:

Press "Del" or click on the "Delete" button:
