how do I move my thing to another level?

I made my mistake to place thing to wrong level
so I want to change my thing from level_a to level_b

how do I do?

If it’s too complex to just drop assets into level b, you can try attaching it to a sublevel then opening that sublevel in level b.

In level A, open the levels window (windows > levels), select everything in the scene you need to move, then in the levels window press the top-left button and choose “add new with selected objects”. Save all. Now open up level B, go back into levels window, add prexisting sublevel.