Layout > Working with Entities > Modifying Entities > To Trim or Extend Line Segments
  
To Trim or Extend Line Segments
To trim or extend segments, hold down the CTRL key and drag the endpoint of the entity to the required location.
You can also trim or extend entities using the buttons on the Design tab:
• Click Make Corner and select two nontangent line segments to trim or extend to create a corner.
• Click Trim Segment and select a line segment to delete.