how to switch photo to landscape

You can use the rotate tool to switch a photo from landscape to Portrait orientation: or vice versa:

Crop, rotate, flip, or straighten photos and videos on iPhone - Apple Support

1. Go to the Photos app  on your iPhone.
2. Tap a photo or video to open it, then tap .
3. Tap , then do any of the following:

• Rotate: Tap  to rotate the image 90 degrees.

You may need to tap three times to rotate clockwise.

Using an alternative interpretation of the question...

If you have a photo that has been recorded with the wrong orientation, so that streets are vertical and the sky is on one side, you can rotate the picture in edit mode. (In the current iOS, it is part of the crop/rotate function.)

If the photo was already taken, you don't. If you're asking how to take photos in landscape mode, just turn the phone sideways.