how to stretch part of object How can I stretch a part of an object ?

I selected one side of the area I'm hoping to stretch (just want to add 2/3 cm) but if I press E (extrude) it's going in a weird angle, and don't seems to be the solution anyway.

How should I proceed?
Ok, I found something, but surly is not the right way to do it, also having some issue with this method:


Select one part of the side 
SHIFT + D to duplicate
Delete the primary part of the side you just duplicate
in edit mode select the vertex you want to stretch.
press E then right click to cancel.
on the submenu add the value you want (4cm in Z in my case)
move it back together.


In my case it's not really smooth