You can click on the my stuff tab in the middle of your learntomod homepage, then scroll down to worlds -- that will show you a record of all of the worlds you've downloaded (including any mods or structures that are in that world that you've added and have stuck around). The first world in the list is your current world. Just click the world you want to return to and click the green put on server button.
However, keep in mind that if there are any endlessly looping or problematic mods on your old worlds, those will return as well. But it's easy to fix that, if happens. Just download a new world.
To download a new world (click the what's hot tab then choose a world, click it and press the green put on server button. I recommend ONLY choosing a curated world.