If it has been a while since you played the collection, next time you start the game up you will be prompted to update the game. Even if you have played since that update you may be interested in what exactly that update may consist of.
343 has improved the success rates while searching for matches. They have also fixed the issue where players would sometimes appear to be searching in an incorrect “FIND GAME” screen. The notorious countdown sound effect has been added to the voting timer.
Halo 2: Anniversary network performance has been improved; 343 says specifically in regards to bandwidth spikes. There has been improvements made in the party joining via the in-game roster.
Some players had found the music to be too loud so 343 has taken their own initiative to lower the music in the menus. There have also been updates to the user interface and menus to improve multiplayer status messaging.
If you would like more details other than those outlined above click here, but these were a few of the main highlights. However it is hard to disagree when it is said that these improvements were much needed and will help all of us enjoy the games the way they were meant to be played.