To move a sub-folder to main you have to bring the folder all the way to the top above the inbox folder. It'll say move folder to base (main) then it will be a main folder. I have like 27 folders so I had to drag it to many folders to get to the top. Scrolling up each time to get higher in the folder list until I could see the inbox folder then right above it dropped it and it became a main folder again. Hope this helps!