@Barbara-King Hi, can you post a full screen capture of Magix, if you click the Start button - Windows Accessories - Snipping Tool, you can capture the whole screen, post it on here using the arrow button next to the smiley at the top of a new comment,
Are the clips in portrait mode & look wrong, so you want to put them in landscape to loo correct?
You may need to go to File - Settings - Movie, make the aspect ratio 1080 x 1920 instead of 1920 x 1080, or use one of the presets like Instagram Stories, then use the Rotation tool to change the media on the timeline to portrait (vertical)
If yoou select all the clips on the timeline, lasso them with the mouse or ctrl+A, it will change all selected
. . . . How can I change 20 video clips in the film which I am editing, from horizontal into vertical. . . . .
Assuming you mean the video clips were recorded in 'portrait' mode and on being imported are flipped to landscape ie rotated 90° left or right.
Use the Rotation effect to rotate the clips 90° clockwise or anticlockwise depend in how they imported.
If you want the output to be in the same orientation, see this tutorial.
if you are mixing Portrait and Landscape video or images then you have to make several decisions:
Decide whether the final output is Landscape or Portrait - set the project setting to this - you cannot switch from one to the other in a video export.
Accept the pillar or letter boxing - ie black bars left/right or top/bottom
or
Crop (Section effect) the clips/images to fit if you wnat them full screen
Note: - if these video clips are from a mobile phone you may run into other issues which have been covered in other topics in the forum - forum Search function should find them if you do have other issues.