Deleting a sim is really quick and easy, all you need to do is click on the sim you want to delete from the game (when they aren’t completing an action) and you will see a few options. On the right hand side of your sims name you will see their picture and a red circle with a cross through it, this is the delete button:
Click on this and it will ask you if you are sure you want to move this sim out of town (which means you will remove them from the game):
If a sim is working towards upgrading a life orb when you delete them they will leave behind the orb they started with, for example, if you give a sim a bronze life orb to turn into a silver but you delete them before they have a chance to upgrade the orb they will leave behind the bronze life orb. If they were working towards a bronze life orb they won’t leave behind any orb.