https://www.washingtonpost.com/politics/2024/07/21/open-convention-democrats-biden-drop-out/

“How Democrats would pick a new candidate if Biden drops out, step by step”

One [possibility] is a virtual vote that would lock in a new nominee in early August, and the other is an “open” convention, a scenario the party hasn’t experienced since 1968.
A convention is open when no candidate arrives with a clear majority of delegates, so the event turns into a mini-primary in which contenders scramble to persuade delegates to vote for them…
Some states have August deadlines to get on the ballot for the general election, and early voting begins in some places in September. So party leaders probably would try to settle the nomination before the Democratic National Convention begins Aug. 19.

There are two types of Democratic delegates. Pledged delegates commit to supporting the candidate state voters chose, although a “good conscience” clause in the party’s rules gives them a bit of wiggle room.

Automatic delegates, often called superdelegates, are the party’s highest-profile leaders. They have the role because of the offices they hold (or held), and the group includes former presidents and vice presidents, Democratic governors, members of Congress and party officials. They are not pledged to any candidate and are not allowed to vote on the first ballot at the convention.