Two changes would happen. A lot of people who don't vote today because their state either heavily favours the 'wrong' candidate or the 'right' candidate will already win by a massive margin, would start voting, since all of sudden their vote matters a lot more.
Secondly, and more importantly, candidates would campaign very differently. In todays system gaining 1-2 percentage points in swing states like Arizona, Georgia or North Carolina is worth infinitely more than gaining 5 percentage point in California, New York or Massachusetts, and politicians on both sides campaign accordingly. In a popular vote scenario those extra 5 points in California could swing the election which would change the entire focus of not only the campaign, but possibly even the type of policy and even candidates that the parties put forwards.