I have downloaded an app for Messages, and I'm wondering how to delete it. I don't see it on the iPhone.
If it's an app that's ONLY for Messages (meaning there's no supporting app that is accessible from outside Messages), you uninstall it by going into Messages, taping the A application icon, tapping the four bubbles in the bottom left corner, then swiping through pages until you find it. Press and hold on the app's icon and an X will appear. Tap the X to uninstall it.
If it's part of a larger iPhone app, for example a game that includes stickers, you would delete it by deleting the iPhone app itself.