Nikon D6 Reference Manual page 1027

Straighten
Rotate pictures by up to ±5° in
increments of approximately 0.25°.
•
The effect can be previewed in the edit
display.
•
The greater the rotation, the more will
be trimmed from the edges.
• Press 4 or 2 to choose the amount of straightening
performed.
• Press J to save the retouched copy.
G button U N retouch menu
N The Retouch Menu: Creating Retouched Copies 975