This has been my go-to app for years for hiking and backpacking. There is a known issue for 6s users as documented on the authors web site. The problem is touching on a grid square to prompt for downloading a map does not work with 3D touch. If you encounter this issue, goto Settings->General->Accessibility->3D Touch and toggle to Off. Now the app behaves as before and you can download new topo maps by touching the grid. When finished, toggle 3D touch back on. Happy hiking!
Hey_aj about Topo Maps, v1.15