Here’s a quick sample I came up with. First, you want to set a menu with the options for your levels, you can name those however you want (you’ve already done with this step)
After that, you need to add the <menu> on A pressed with <selection> <selectedIndex> block from the menu extension (see below). Here, you define what happens when you press a button while in the menu (in this case the A button but you can set it to be whatever you want)
Once that’s done, inside of said block you have to write some code using the logic blocks. You can get which index we’re on when we press the button with the selectedIndex variable (you can drag it out of the block). For example, if the ‘Level 1’ option is selected, the index is 0 (because the index starts at 0 and it counts up), so for ‘Level 2’ the index is 1.
So in the first logic block, you set the tilemap for the first level, in the second for the second level, you get the idea. At the end just put a close menu block because you probably don’t want the menu to be open when you’re playing.
And that’s it! Hope this helps, if you need more help just ask! 
Here’s the link to the project so you can see it for yourself if you need extra clarification.