Since the Republicans are more pro-early voting this cycle (even with Trump), it is tough to say how much of this is actually for Harris, but my naive guess would be about 2/3rds of the early voting is for Harris.

Early voting helps for the President in at least three ways:

1) It makes it easier to do ground game on election day, since more people are already taken care of.

2) It helps with people who end up dying before the election. This is a surprisingly large number. I'm not sure how many this is but here's a rough estimate for how many. A little over 3 million people die in the US yearly, which gives us around 250000 who die with a chance to vote early before they die (assuming about one month of early voting).  Some of those are going to be not eligible to vote (such as too young) but the vast majority will be of voting age. About 2/3rds of all eligible Americans vote, so that's around 150,000 extra voters (but a bit less since not all states allow that much early voting). About 20% of the US population is in swing states, so that's around 30,000 voters. We don't know the percentage breakdown of Democrat of Republican, but if 2/3rds or so of early voters are Democrats, then that's 10,000 extra votes in swing states. Since Trump is aiming at young males, and that's a population with extra deaths by misadventure, this may be an undercount. 

3) Voters who vote early are not going to just forget to vote or have other problems. I've seen estimates somewhere between 0.5 and 3% of people who say they are going to vote forget to do so. This is probably going to hurt Trump more than Harris, because a lot more of his voters he is trying to activate are not regular voters. But forgetting is not the only thing that happens here. People have all sorts of emergencies or take care of problems with friends. For example, there are around [130 million ER visits yearly](https://www.cdc.gov/nchs/fastats/emergency-department.htm), which means one should expect around 300,000 visits on election day. Most of those will be simple and people will get out fast, but if it happens at the wrong time of day, such as the late afternoon before one has voted , or if the visit ends up being something serious enough to take up the whole day, one will lose the chance to vote.  How many this is tough to say, but presumably also in the tends of thousands as an advantage for the Democrats. 
Aside from these advantages at a Presidential level, which will matter in a close election, there's  some evidence that early voters are more likely to vote in down ballot races or referendums. So early voting more also helps with the house and senate. 

TL;DR: The advantage for early voting isn't going to make the Democrats win the election by itself, but it definitely helps.

(Edits for formatting so not a giant wall of text.)