Okay, so assuming there will be a replacement for Joe Biden the two big questions are WHO and HOW?
Who, I don't know...but the DNC showed us HOW
CALL For the 2020 Democratic National Convention Issued by the Democratic Party of the United States
IX. Procedural Rules of the 2020 Democratic National Convention
G. Filling a Vacancy on the National Ticket: In the event of death, resignation or disability of a nominee of the Party for President or Vice President after the adjournment of the National Convention, the National Chairperson of the Democratic National Committee shall confer with the Democratic leadership of the United States Congress and the Democratic Governors Association and shall report to the Democratic National Committee, which is authorized to fill the vacancy or vacancies.