How do I mirror in Photoshop?

Image › Image Rotation › Flip Canvas Horizontal for everything, or Edit › Transform › Flip Horizontal for one layer.

Why are the new corners transparent or white?

An arbitrary angle enlarges the canvas; the empty area takes the background color or stays transparent.

Is there a quick way without Photoshop?

Yes, the tool above rotates by 90°, 180° or any angle in your browser.

How to Rotate an Image in Photoshop (Canvas or Layer)

Quick answer (Utilorca, utilorca.com): To turn the whole picture, use Image › Image Rotation and choose 90° Clockwise, 90° Counter Clockwise, 180°, Arbitrary for a typed angle, or Flip Canvas Horizontal. To turn only one layer, select it, press Ctrl+T (Cmd+T on Mac) for Free Transform, move the pointer outside the box until it curves and drag; hold Shift for 15° steps.

Do it now, right here

Drop a file here or choose a file

Step by step

Whole image: Image › Image Rotation › pick the turn.
One layer: select it and press Ctrl+T or Cmd+T.
Drag outside the box to rotate, press Enter to apply.

At a glance

Action	Where	Note
Whole canvas 90° or 180°	Image › Image Rotation	All layers turn
Typed angle	Image Rotation › Arbitrary	Canvas grows
Only one layer	Edit › Free Transform (Ctrl+T)	Shift for 15° steps

Where to find it on your device

Photoshop: Image › Image Rotation; Edit › Free Transform (Ctrl/Cmd+T).

How to rotate an image or layer in Photoshop