He was for it before it was against it:

Up until now, Trump has pretty consistently insisted that he has nothing to fear from the Mueller report, saying that there’s “NO COLLUSION.” Just a few weeks ago, White House press secretary Sarah Huckabee Sanders said: “We feel good about the fact that what we’ve said all along for last two years will be clear — there was no collusion.”

It is unclear why Trump changed his position on the report.