Every phone user has been there: you shoot a burst of photos, and one of them comes out sideways or upside down. It happens because of how the camera stores orientation data, and it always seems to happen to the shot you actually want to use.
Drop the photo into a rotate image tool, pick clockwise or counterclockwise, and download. It takes about three seconds. No need to open Photoshop for this, and no need to upload your photo anywhere, because the whole thing runs in the browser.
Rotating turns the image, like turning a piece of paper. Flipping mirrors it, like looking in a mirror. They solve different problems. A selfie that looks "wrong" is usually a mirror issue, so flipping it horizontally gives the true image. A photo that is genuinely sideways needs rotation. My image rotator supports both.
Rotating and flipping do not recompress your image, so the quality stays exactly the same and the file size barely changes. That is the honest advantage of doing it this way instead of re-saving through an editor.
Once the orientation is right, you will often notice the photo is too big or the wrong shape for where it is going. Run it through the image resizer to set exact dimensions, or use the square crop tool if it is heading to a social feed.