I'm Japanese and I'm using a translator, so the text may not be correct, but please help me!

When I use a skill called "nether biome", it turns into a nether on a large scale and I don't know how to bring it back.

I've managed to convert the biome with worldsets, but is there any other way to get it back?

Is the spell "netherize" from the "netherside" example?

If so your best is probably to use Rollback, that is a spell from the engineering example. You can either add that example if you don't have it already, or just install that one spell.

/mexample add engineering
/wand add rollback

Casting rollback at the modified blocks should restore them.

Rollback was not possible.
Here is the magic I used.

Oh yeah literally changing the biome ...

There will be no way to undo that properly. Your only recourse is to reset the biomes on all effected blocks to something else. Here's an example using plains:

resetbiome:
  icon: stone_pickaxe{CustomModelData:18001}
  icon_disabled: stone_pickaxe{CustomModelData:18002}
  actions:
    cast:
      - class: Sphere
        actions:
          - class: ChangeBiome
  parameters:
    target: block
    allow_max_range: false
    range: 32
    radius: 128
    type: sphere
    check_destructible: false
    biome: plains

You can copy that into a file in your spells folder if you want.

After copying it
What do I need to do within Minecraft?

/magic load
/wand add resetbiome

Then cast it anywhere.

You could also just use WorldEdit for this, //setbiome or something I think.

Try opening up the file plugins/Magic/spells.yml

delete ALL the contents of that file (should just be comments)

then paste in the spell config from above and try the commands again.

/wand add resetbiome
I did indeed load and execute the command.
But I got this message

I've run //setbiome but it doesn't fix it.

I was able to adapt successfully, but the "reset biome" is not activated.

Does that mean you fixed it?

/ wand addresetbiome. I was able to adapt this one, but when I activate it with the wand, nothing happens.

Biome changes require a relog, sometimes a server restart.

Beyond that I really don't know, sorry. Especially if WorldEdit can't do it.