This one is pretty broad.
Most importantly, the AI will now a one-time launch retaliatory attack when you destroy each core guard post (unless Lazy AI is on). Given that your mobile forces are probably entirely engaged on the AI HW at the time, and that the attack can come from one or more warp gates pretty much anywhere in the galaxy, this could be... uncomfortable ;) You could of course pull your mobile ships back before actually landing the kill shot on a core guard post, but that takes time during which the normal AI response has time to work (and each core guard post you kill increases the AIP floor by 10, so even if you started the assault at low AIP you may find you don't have the luxury of flying back and forth for each core post kill. Or even of waiting for each post's retaliation to be dealt with before killing the next one.
On the other hand, for those wanting a less vigilant AI the Lazy-AI toggle now also disables the "deepstrike" logic which causes threat to spawn constantly while you have military ships several hops deep into purely AI territory. Combined with the other factors this definitely moves the Lazy-AI toggle into the realm of "if you play with this, balance is up to you" in that you'd need to turn up the difficulty in other ways or it's just going to be a lot easier than a "normal" game, but it's there for those of you who like getting your challenge some other way or simply like trying to win with only one your HW or whatever.
Another important change is that now the AI's normal reinforcements stop getting stronger past a certain (difficulty-based) AIP threshold. We may need to add some compensation for this later (intensifying its offensives past the threshold, probably) but for now the idea is that if you cross into the AIP "death zone" it should just kill you via some offensive mechanic rather than stonewalling you with a huge wall of entirely defensive ships that regenerates faster than you can kill it. Stalemates like that have a long and storied history in AIW, and that's not the end of the world, but many people have complained about that over the years and if we can move towards something that's more likely to avoid those (in favor of moving the game towards a conclusion one way or another) then that's probably a good thing.
This release also contains several bugfixes and other balance changes, all mentioned in the notes linked above. Most notable of those is a fix for the longstanding bug with the "AI units should wait at least 30 seconds before retreating" rule not working. Now your chipper shredders will have a little more time to work!
Update: 6.020 hotfix for a bug in recent versions where scouts could not be directly sent through unexplored wormholes when the everything-starts-unexplored toggle was on.
Update: 6.021 hotfix to fix a bug in the AI-retreat-behavior "fix" that was causing errors on the AI thread.
This is a standard update that you can download through the in-game updater itself, if you already have 4.000 or later. When you launch the game, you'll see the notice of the update having been found if you're connected to the Internet at the time. If you don't have 4.000 or later, you can download that here.