The Mr. President 2nd Edition Update Kit will include the following items:

6 cards
16 counters (duplicates of those in the 2023 Replacement Countersheet—note that four of these are very minor errata; the other 12 are helpful reminder counters)
4 double-sided Action Cards (Domestic, Diplomatic, Military, Presidential)
Booklets:
Scenario Booklet ~36 pages
World Peer China ~12 pages
World Peer Russia ~12 pages
Ally/Rogue State ~12 pages
World Charts ~32 pages
Governing Manual ~52 pages
Domestic Charts ~20 pages
Flipbook ~28 pages
Briefing Booklet ~20-24 pages.

You'll note that's most of the Booklets and PACs in the game! Some of this is to accommodate errata and clearer wording from the FAQ for the 2nd Edition, but it will also include numerous game enhancements to make the game more challenging (especially in years 3 and following) and to add features, importance, and more difficult choices to areas like:

Ally Relationships/Actions
Strategic Capabilities
Combat (Counterattacks, Carrier Support and Combat DRMs based on Strategic Capabilities, Heavy units less useful in Guerrilla War)
Iran at War (including a new Action Table)
Cascading Events (January Surprise)

The Update Kit is suitable to use as a VASSAL/TTS Play Kit.