The incorrectly spelled word is "Magnificient." The correct spelling is "Magnificent."

Here's an explanation for each option:

1. Hilarious: This is the correctly spelled word. It means extremely funny or causing great amusement.

2. Demolish: This is the correctly spelled word. It means to completely destroy or dismantle something.

3. Magnificient: This is the incorrectly spelled word. The correct spelling is "Magnificent," which means exceptionally beautiful, impressive, or grand.

4. Gargantuan: This is the correctly spelled word. It describes something of immense size, magnitude, or scale.