In a 2D sketch, extends a curve to the nearest intersecting curve or as defined by selected boundary geometry. Creates a coincident constraint between the endpoint of the extended curve and the boundary curve.
In a 3D sketch, extends curves to intersect with a face, work plane, or surface. 
Access
Ribbon:
Sketch tab
Modify panel
Extend
You can extend curves with or without defining boundary geometry.
Pause the cursor over the curve to preview the extension.
Dimensional and geometric constraints affect the results of the extend operation.
To change to the Trim or Split command, right-click and select the appropriate option from the context menu. To switch temporarily to the trim operation, hold down Shift. The extend operation resumes when Shift is no longer pressed.