As it says on the tin. Somehow get the Woodtock music festival to be the three whole days of peace love and joy that its promoters had been hyping, and Altamont to be the violent debacle that everyone had been predicting in the wake of all the beatings and bloodshed at Yasgur's farm a few months earlier.
Also, what does this do to the respective reputations of the musicians and genres involved? In OTL, the Stones got a bit of a feather in their collective cap for managing to pull off a far more civilized event than the supposedly peace-loving hippies had. Mick successfully calming things down in the few instances where things threatened to get a little hairy is an especially fond memory of the era.