Use ⌘-R or Reload from the Web-App menu to reload the Web-app from its base URI. If you JUST want to reload it from the currently displayed page, then right click on the page and select Reload.
Psst. Hey, if you want to reload ALL of the Web-apps, select Reload All from the Web-app menu or hold down ALL of the modifier keys (Control, Option, Shift AND Command) and then press R.