If you don't have a cover photo yet, you can add one by going to your group and clicking on "upload" at the top left of the photo location.
If you want to modify your cover photo, it is the same manipulation but the icon at the top of the photo will show "update cover picture".
A folder will then open so that you can choose your photo.
You can then resize/center it by clicking on the icon at the bottom right of the photo or delete it by clicking on the small trash can at the bottom right.
Please note that the cover photo must be large enough to fit in the frame without being distorted. We therefore advise you to choose a photo taken in landscape mode, especially if it is a photo from your phone.
Web version :
Mobile version :