How do I remove the top sash of a medium size new double glazed window?
Find the Mechanism
There is usually a release mechanism along the top edge or the bottom edge of the upper sash that allows it to tilt in.

Get the Lower Sash out of the Way
The trick is moving the inner, lower sash out of the way.  That is done by releasing the mechanism on the lower sash, tilting it inward and lowering it toward the sill. 

Tilt the Upper Sash in
You then slightly lower the top sash, which is no longer blocked by the lower sash.  You can then release the mechanism on the top sash and tilt it inward, either up or down depending on the position of the mechanism.