Quote: "There's (currently) no way to do this."
Technically, YES, there is no way to go back a level.
You can give the "illusion of returning" with a brand new level Prefabed from the first.
If you made a level 3 that was a carbon copy of level one (prefabs come in handy), then
you can move the player's start marker to the end of the level for coming back in.
You could also place new scripts for the entites to behave differently or the same, etc..
But he's right, you can't go back and forth like navigating a web browser.
It always moves forward.
I am currently using this method to have a "Briefing Area" as the first level of my game.
Once, you have completed all the battle levels of your mission, then you "return" to the same "briefing area".
This time instead of being instructed on your mission, you are congratulated for completing it.
The same commanding officer that "briefed" you, now says well done, and then rolls the credits.
That's my advice, use a prefab and emulate your first level.
It's the only way to "return" to a level.
Whatever you can imagine, you can animate. --- Walt Disney
All too easy. --- Darth Vader
Just do it! --- Nike