Providing the layer itself is not locked, the contents of each layer in your Photoshop documents can be individually manipulated without affecting the rest of the image. This includes rotating the entire layer, a task that is accomplished by using commands found in the Transform submenu.
In the Layers pane, select the layer you want to rotate. If you need to rotate multiple layers in the same way, you can select all of them and rotate them simultaneously instead of doing each individually. Click "Edit," hover over "Transform" to display the submenu and select the command that is most appropriate for your situation. Three commands, "Rotate 180," "Rotate 90 CW" and "Rotate 90 Counter Clockwise," rotate the selected layer, respectively, by 180, 90 clockwise and 90 counterclockwise degrees. Select the "Rotate" command to freely rotate the selected layer by any angle you desire.
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After selecting the "Rotate" command, you will see a bounding box around the contents of the selected layer. Move your mouse cursor outside this bounding box and it turns into a curved, two-sided arrow. Click and drag to rotate the layer; if you hold "Shift" while doing so, Photoshop constrains the rotation to increments of 15 degrees. By default, the layer rotates around its center. To change this behavior, drag the small crosshair icon from the center of the bounding box to the position around which you want the layer to rotate. After you're done, press "Enter" to confirm; if you change your mind, press "Esc" to cancel the rotation altogether.