You can rotate a photo directly in the Photos app, so you do not need a separate image editor for a picture that is sideways or upside down.
- Open Photos and select the picture you want to rotate.
- Tap Edit.
- Tap the Crop button.
- Tap the rotate button to turn the image 90 degrees. Tap it again if the picture needs another quarter-turn.
- Tap Done when the photo has the orientation you want.
Photos keeps the edit reversible. If you later want the original orientation back, open the picture, tap Edit, and use the option to revert the photo to its original version.