Open it in any photo editor or image editing package, rotate it, save it, then load it up to display - problem solved.
They come displayed that way cos when you took the pic, you didn't align your phone or camera with the 'default vertical' setting you have saved on that device, so it's being displayed according to the way your device was set at that time - the way the device was set by you!! You could probably rotate the image in your device's gallery & then save it again in order to get it to display the way you really want it to appear; or you could
always select portrait or landscape mode in your phone/camera as appropriate whenever you flip the device from straight up onto its side - well, unless you WANT the pic to display sideways....
It took me ages to work out what I was doing to cause this, but once I realised my devices have a 'default vertical' & that I should change that whenever I swap the alignment from portrait to landscape or vice versa, my pics started appearing the right way up when displayed!! :thumbup: