Video Edit, How To Rotate To Play On Tv

[CW1]

Some videos I've taken on my phone play absolutely fine on my PC but need rotating 90° to play on my tv. Don't understand why, but is there an easy way to edit the mp4s so I don't have to tilt my head to watch on tv. I can rotate on the PC but can't save.
Would it mean I'd need 2 copies of the the files, 1 that plays on the PC, & another for the tv ?

[Buenchico]

There are lots of free online services for uploading and rotating video files.  (Simply google 'rotate mp4' to find them).  However you need to be a bit wary when it comes to using them, as some (most? / all?) might want you to download something which includes some form of malware.

A better option would be to install a free video editor, such as Capcut:
https://www.capcut.com/
and then follow the instructions here:
https://www.capcut.com/resource/how-to-rotate-videos