If you want to delete a person, click on said person and then (on the left side of the screen) select the button "More Options". This will lead you to, well, more options, and one of these is "Delete person".
However, this only works/the option only appears if the tree is going to stay "intact" without this person. If you want to delete - for example - your father's sister, that's no problem. But if this sister has a wife, you'd need to delete the wife first (as the wife would otherwise not be connected to the rest of the tree anymore). Same thing goes for a person who is the sole parent of a child that wouldn't have a connection to other people on the tree without this parent. To delete these people, you'd need to delete the people dependent on them (i. e. kids and partners) first before deleting them.
