Assuming that the clip has a 9:16 scale and you project a 16:9 scale (see the scale at the bottom right of the preview screen), rotating is a very easy action: Draw the clip from the media room onto the timeline. Select the clip by clicking on it. In the preview-screen you will see the clip and in its 4 corners white dots and in the middle a grey circle with arrow that turns white when you hover over it. So, hover over it and when white drag the picture with the mouse either to the right or to the left so that the picture turns 90 degrees.
Once you have turned it so that it is in the right stand, take one of the white corner dots and drag it to the corner so that the image takes the whole screen. You may see pink lines as soon as the dot is aligned with the border, but make sure that all picture borders line with the screen border, so that there are no black borders when you produce the clip.
The alternative to the above is: using the PIP designer, double click the video clip in the timeline. Make sure that you are in advance mode (depending on your version) that can be changed from express in the middle above the preview screen of the PIP designer. At the left you find all sorts of options including the rotation, depending on the clip rotate either 90 degrees or minus 90 degrees (or use the grey/white circle in the middle again. And adjust the size of the clip by dragging the white dots to the correct position.

The results is good, but you will remember probably that holding your phone in the correct position the next time is even better :

And as always: look for YouTube tutorials that explain the above: like those of Maliek, and many others.
Have fun

Quote Assuming that the clip has a 9:16 scale and you project a 16:9 scale (see the scale at the bottom right of the preview screen), rotating is a very easy action: Draw the clip from the media room onto the timeline. Select the clip by clicking on it. In the preview-screen you will see the clip and in its 4 corners white dots and in the middle a grey circle with arrow that turns white when you hover over it. So, hover over it and when white drag the picture with the mouse either to the right or to the left so that the picture turns 90 degrees.
Once you have turned it so that it is in the right stand, take one of the white corner dots and drag it to the corner so that the image takes the whole screen. You may see pink lines as soon as the dot is aligned with the border, but make sure that all picture borders line with the screen border, so that there are no black borders when you produce the clip.
The alternative to the above is: using the PIP designer, double click the video clip in the timeline. Make sure that you are in advance mode (depending on your version) that can be changed from express in the middle above the preview screen of the PIP designer. At the left you find all sorts of options including the rotation, depending on the clip rotate either 90 degrees or minus 90 degrees (or use the grey/white circle in the middle again. And adjust the size of the clip by dragging the white dots to the correct position.

The results is good, but you will remember probably that holding your phone in the correct position the next time is even better :

And as always: look for YouTube tutorials that explain the above: like those of Maliek, and many others.
Have fun

Hey thanks a lot! Buddy I really appreciate that! That saved me days of looking how to do this.

Appreciate it!

Quote
The results is good, but you will remember probably that holding your phone in the correct position the next time is even better :

This way, the zoom will reduce the quality of a HD video to the levels of a 1080x607 pixels video, barely above SD.
I would not call that "good", but only "acceptable".
That's why holding the phone correctly is so important!

Fully agreed!

(I had in mind the 4K I have set as default, which gives a bit more acceptable result.)