Do you mean to mirror it?
If blockland still has support for mission editor you can flip the build using it.
I looked around in mission editor, but I couldn't figure it out.
I'm a bit rusty on this so I might be wrongPress F3 and select every brick then in one of the boxes enter 180 to flip it halfway.
How do I select bricks?
Scale by -1 and move the bricks so they're back in brick grid. It's probably not going to work but apart from manually building it this is your best chance.
You could write a script to flip all of the positions in the save file.