I have a very old iPhone, and I have to admit the only thing I use the iPhone app for is to upload my photos (it is just very easy to do it directly from my photos). This is about the only thing that is easier to do on the app than on the website. As Star3 suggested, for everything else I go to the website using a large screen computer.
If you want to use the iPhone, though, you can tap the observation, then tap the middle button on the observation (… in a speech bubble) and then hit the green “suggest ID” at the bottom. It will take you to the screen where the algorithm makes suggestions, or you can type something in the box. Then save it (upper right). It will cross out your previous ID and put in the new one.
To edit the location, click the “Edit” button in the upper right, then look about midway down the screen where it put the location, right after the date. If you click that location, a map with crosshairs comes up and you can move the cross-hair to a new spot. Don’t forget to save the location change, and then click “done” (Upper right).