You may be asking what happens now.
There is a process for when a candidate withdraws.
The League of Women Voters of the United States, as a member organization of the National Task Force on Election Crises, released the following statement on the Democratic party replacing its presumptive presidential nominee:
"While a major party’s presumptive nominee withdrawing his presidential candidacy is an extraordinary occurrence, rules and procedures are in place to respond to this development.
"Americans can be confident that this situation falls well within existing rules and that their election system continues to allow for a free and fair election.
"A political party has control over how to choose its nominee in accordance with the party's own governing rules. When a presumptive nominee withdraws before their party makes its nomination official, the party will follow its own rules and procedures to choose an official nominee (typically, but not necessarily, in person at the party's nominating convention). In this instance, the Democratic Party will determine its path forward, and its delegates will vote to approve a nominee at or before the Democratic National Convention this August.
"Additionally, state ballot access laws are no obstacle to the party choosing a nominee other than President Biden. States have not yet printed ballots for the general election, and state laws across the country provide for the candidate chosen by the Democratic Party at or before the convention to appear on ballots this November irrespective of whether that nominee is the same individual as the party’s presumptive nominee based on the results of the party’s presidential preference primaries."
Read more about candidate succession from the National Task Force on Election Crises -- Here