how to delete a folder in Photos

Just to make sure: you said "folder" and Christiaan referred to albums. On a mac a Folder holds Albums, and Albums hold pictures. You can't put a picture into a folder. But you can put a picture into an album and put the album into a folder.

To delete an Album you "right-click" or "control-click" or "two-finger-click" the album icon, and then choose Delete Album. This won't delete the pictures inside the album-- the pictures aren't really inside, so it's just the Album name and icon that disappear.

To delete a Folder you "right-click" or "control-click" or "two-finger-click" the Folder icon, and then choose Delete Folder. Again, this won't delete any pictures, but it will delete all the Albums inside the Folder.

It depends on which platform you are asking this question for. On iOS, if you want to delete an album, you slide to the left on the album and you get the option to delete the album. On a Mac you click on the right mouse button (by this I mean the sub menu of the album) and you can choose there to delete the album.

I hope this connects to your question. Otherwise, feel free to let me know.