Highlights:

• 2 newly added battle stages (Barnacle & Dime and Humpback Pump Track)
• 1 new Salmon Run stage (Jammin’ Salmon Junction)
• 11 newly added weapon sets (including 2 new main weapons: S-BLAST ’92 and Painbrush)
• New Challenge mode
• A new seasonal catalog
• New music
• New and returning stuff in the shops

Patch Notes (EN): https://en-americas-support.nintendo.com/app/answers/detail/a_id/59461#v400

Patch Notes (JP): https://www.nintendo.co.jp/support/switch/software_support/av5ja/400.html