Go to the LearnToMod homepage and select a world to download. Click the 'What's Hot' tab in the middle of the homepage then scroll down. Choose any worlds located under the heading 'Worlds' and to the right of the Worlds search button. This will clear any out of control mods and start you off fresh. This is also useful if you need to respawn for other reasons!
(Note: You can always get your old world back; just go to My Stuff < My Worlds)
Exit out of your game or Minecraft launcher. Then go to the Server Admin page. The server will need to be stopped and restarted before it'll render your new world - this means you'll need a new ip. Either wait long enough for the Server Admin page to lose your ip (~15 minutes) or press the Force Shutdown button. If the Force Shutdown doesn't appear to work, try exiting the browser tab then re-entering the page.
Finally, download the world by pressing the put on server button, Wait 1 to 2 minutes and log back into Minecraft.
If the world map doesn't render right away, you can type "dynmap fullrender world" in the command tile or "/dynmap fullrender world" in the game. You can set the weather and time of day in your world from the Server Admin page, as well - try it out!