There is however a workaround to rotate a video in the new Youtube Studio (even after the Classic Editor button was disabled by Google).
It is a bit tricky and I don't know how long it will work.
Here is the way to do it:
- Open new Youtube Studio and select the video to edit.
- Note down/copy the <video-id> of your video to rotate.
- Create a url with your <video-id> as follows: youtube.com/enhance?v=<video-id>
- Open that url in a new tab.
- The old enhance edit window appears (which was part of the classic studio)
- However, you will only see a "Trim" button.
- Right click the "trim" button and select "inspect"
- Select the line below the highlighted one in the inspect window (should be something like: <div class="enhance-effect" id="enhance-rotate-buttons" hidden="true">)
- In that line: highlight "hidden="true"" and delete it and hit return.
- Now you will see the rotate left/right buttons again.
- Rotate your video and click "Save" or "Save as new video".
This may take some time.
Source: https://www.quora.com/Is-there-a-way-to-rotate-a-video-on-YouTube
