To change your main image on the mobile site or app please visit Main Menu >> click the Pencil icon in the top left corner near your profile photo >> Tap on the image you want to use (if it is approved already) >> Tap the menu button (the three horizontal dots in the top right corner) >> select 'make main photo.'
Please note that if you try to move a restricted image from your private album to your public gallery, you will be advised that you cannot. You cannot have a restricted image as a main photo.