Once they get the candidate situation situated, donate and canvass for them. Convince your friends to vote. If you feel voting doesn't matter, remember there's more on the ballot than the presidential race. A lot of the time meaningful change starts at the local level. You'll see your political power in action if you get involved in your community.
Information on Canvassing.
Harris had a meeting with the financial backers a few days ago. It would probably be easier for the funds to go to her campaign then if someone fresh started. One of the arguments for Biden to stick around was that the new candidate would have to raise nearly a billion dollars to even have a chance at winning.