Just after the confirmation of Generals Mattis and Kelly for Trump's cabinet, he signed an executive order concerning the ACA. Unfortunately, we don't know what it says.
I checked the White House website to try and get the text of the order, but this is what it looks like, 30 minutes after the order was signed.
There is some speculation.
Apparently it is the closest he could get to keeping his promise to repeal it on Day One.
Update: Here's an image of the order. The text is still not up on the website.
Basically it directs agencies not to enforce taxes, to work with states on whatever and however they feel like implementing it, and generally to treat it with neglect.
Update 2: Some are saying this effectively guts the individual mandate. I guess that's true, in the sense that if the IRS doesn't enforce the collection of the tax, the teeth come out of the mandate. But it's a little bit dramatic to go that far, given that it's still the law and it's still going to be something reported on tax returns. I'm not sure having an unenforced debt to the IRS for failure to have health insurance is a particularly great position for taxpayers to hold.
Update 3: Sarah Kliff at Vox has a really good article on this. Lots of heat, very little light.